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Aquinas on the individuality of thinking.


THE IMMATERIALITY AND INDIVIDUALITY OF THINKING. To contemporary readers, it seems obvious that a human person is the subject of his thinking. We have been accustomed to the definition of the subject as a thing that thinks. However, philosophers and historians have not tired of warning us that this conception of subject is a late invention. In his formidable approach to the "archaeology of subject," Alain de Libera follows Heidegger and Foucault in identifying a significant transition from the Aristotelian conception of subject as the 0substratum that underlies all sorts of changes, including thinking, to a more familiar notion of subject as an "agent," the active principle of thinking. (1) Unlike his predecessors, de Libera argues that the subject-agent is not a modern creation but rather the fruit of debates over thinking and the self in the long Middle Ages, in which Aquinas's critique of Averroes' doctrine of the unity of intellect made a significant contribution. (2)

Averroes' basic idea is that all human beings share a single intellect, separate from each, which functions as a causal principle of their thinking or understanding (intelligere). (3) A human being can be engaged in an act of thinking only when he is conjoined with this single intellect. Averroes believes that this theory explains why we can think about the same thing without positing Platonic forms. Nevertheless, he also holds that our acts of thinking are still different and individual, because the ultimate principle of thinking, the separate intellect, is the same while its union with us varies from person to person. (4)

To Aquinas's mind, there is a very serious defect in Averroes' theory, which is that it cannot offer a satisfactory explanation for the obvious fact that "this human being thinks (hic homo intelligit)." Aquinas insists that Averroes' claim that this single, universal intellect is the ultimate subject-agent makes it impossible to attribute the act of thinking to individual persons, since they have merely an external relation to the intellect. The external conjunction of a human person with the unique intellect is not sufficient to establish this particular individual person as the agent of his own thinking, because thinking seems to be an activity happening to him rather than an action initiated by him. Instead, Aquinas argues that this human being thinks only when the principle of thinking, that is, the intellect, is an inherent part or power of his soul.

Aquinas's emphasis on the individual agency of thinking seems rather appealing to us, at least prima facie. But it poses serious challenges to Aquinas himself when his hylomorphic conception of the human being is taken into consideration. Following Aristotle, Aquinas conceives of an individual person as a natural compound, with the soul as the form and the body as the matter. At the same time, Aquinas also accepts Aristotle's obscure claim that the intellect is not mixed with the body or the matter, for which reason it may think about all things. (5) As the intellect's activity, thinking is likewise incorporeal or immaterial. In consequence, a dilemma arises in Aquinas's philosophical anthropology concerning the subject of thinking.

On the one hand, if the human being as a whole is the subject of thinking, as implied by the dictum "this human being thinks," Aquinas needs to account for the difficulties in attributing an immaterial act to a being whose nature involves matter. The materiality proper to each human person's nature seems to be an insurmountable obstacle to the coherence of Aristotle's belief that the intellect possesses an unlimited capacity to know all things. For Aquinas believes that the determinate nature of a knowing person limits his cognition, just as a tongue inflected with bitter humor cannot perceive anything sweet. This conceptual difficulty is particularly challenging for Aquinas because the human person is taken as the agent of thinking rather than as the mere substratum that underpins the process of thinking. Even worse, Aquinas uses the immateriality of thinking as the central premise of his arguments for the soul's immortality. The intellective soul can survive death primarily because its thinking is its own operation and does not require the body's participation. To admit the individual agency of thinking seems therefore to undermine both Aquinas's loyalties to Aristotle's noetic theory and his rationale for affirming the power of reason to demonstrate the immortality of the soul even apart from Christian revelation.

On the other hand, if we ascribe instead the principle of thinking to the intellect or the intellective soul alone, as Aquinas appears to have done in his argument for the immortality of the soul, then we seem to threaten the natural unity of human soul and body as a hylomorphic compound. Furthermore, there is a deeper ontological problem concerning the individuality of thinking. For Aquinas seems to commit himself to the Aristotelian theorem that matter is the principle of individuation. If the active principle of thinking is taken as an immaterial power, then how can it be individuated? How is the individuality of thinking related to the individuality of each human person? Are Socrates and Plato still distinct from each other in their thinking? If so, how?

What concerns us here is the metaphysical possibility of "this human being thinks" within the framework of Aquinas's philosophical anthropology. Whichever horn of the dilemma Aquinas takes, the ultimate problem that confronts him is the compatibility between the immateriality of thinking and the individuality of thinking. In other words, for a medieval follower of Aristotle like Aquinas, it is not unproblematic to assume that an individual person is the thing that thinks, no matter how clear and distinct the person's inner experience that it is he himself who is thinking. (6) However, as his contemporary adversaries complained, Aquinas seemed to be satisfied that the empirical evidence sufficed to refute Averroes since Averroist noetic theory failed to save this obvious phenomenon. (7) Most of recent literature follows this criticism to challenge the consistency of Aquinas's position, in particular his conception of the human intellect as the sole principle of thinking, which seems to be irreconcilable with his claim that "this human being thinks." (8) Even Alain de Libera, after affirming that Aquinas introduces a "modern" concept of subject-agent by identifying the human intellective soul as the subject of thinking, (9) also carefully reconstructs Siger's arguments to show that Aquinas is incapable of explicating how the act of thinking is an operation of the whole human being as a composite of soul and body. (10)

This essay aims to address these challenges by reconstructing the ontological reasons Aquinas could have offered to demonstrate the compatibility of immateriality and individuality of thinking in his controversy with the Averroists. The challenges are twofold. On the one hand, it is necessary to show that the individuality of an embodied person will not jeopardize the immateriality of his thinking nor the possibility that someone else could think about the same thing. On the other hand, one also needs to show that intellectual realities like thinking can be individualized just as the existence of a human being is individualized. In other words, to resolve these challenges to the coherence of Aquinas's position, it is necessary to show how Socrates' thinking can be distinct from Plato's thinking even when they are thinking about the same thing. After resolving these challenges, we can then move on to clarifying the conditions under which thinking can be identified as the action of a single human person. Only after we have shown that a human person can think can we defend the claim that "this human being thinks."

However, the metaphysical approach to the individuality of intellectual thinking has not been sufficiently appreciated by commentators on Aquinas's arresting dictum hic homo intelligit. (11) Having rightly detected the tension between the immateriality of thinking and the apparent materiality of a thinking person in this claim, most scholars tend to be satisfied with identifying the ambiguous status of the human intellect as the only solution Aquinas can offer. For Aquinas, the human intellect denotes both a power of the soul as the immediate principle of thinking and the human soul itself that informs a material body. Since Aquinas recognizes a real distinction between the soul and its powers, it seems to be possible for the human intellect to be both an immaterial power and a material form. (12) Putting aside Aquinas's controversial distinction between the soul and its faculties, this solution has to face another problem: How can the principle of thinking (the intellect as a faculty of the soul) be individualized and become a power of the form (the intellect as the soul) that is individuated by the animated human body? (13) One may appeal to other suggestions such as Aquinas's claim that the human soul is a form that is not entirely immersed in the matter and therefore can have an immaterial power like thinking or the principle of actiones sunt suppositorum to argue that even though thinking is an immaterial action, only a human person as a suppositum, or an individual subsisting in the genus of primary substance, can be its genuine agent. (14) But as I shall argue in the following pages, the same problem arises again: How is it metaphysically possible? Only after clarifying the ontological foundation for individual thinking can we adequately respond to those critics of Aquinas, both medieval and modern, who argue that his noetic theory is not consistent.

This essay will attempt to show how Aquinas's account of the immateriality and individuality of thinking can withstand the arguments of his critics. First, I will revisit Aquinas's accusation that Averroes fails to account for individual thinking to examine Aquinas's own metaphysical presuppositions. This approach will give us a more vivid picture of the tension between the immateriality required by Aristotelian epistemology and the individuality seemingly implied by our own inner experience of thinking. Then I will reconstruct three significant ontological presuppositions from Aquinas's texts that indicate a way to demonstrate the compatibility between the immateriality and individuality of thinking. The first and most significant of these presuppositions is Aquinas's original conception of individuality in terms of imparticipability, which allows him to establish the individuality of thinking without reducing the intellectual soul to a material form. For even with material beings, matter is not the ultimate principle of individuation. (15) The second presupposition is concerned with the complicated status of the intelligible species. The intelligible species is an individual form in terms of the mode of existence, but a universal form in terms of its content. The mechanism of intelligible species helps Aquinas explain how the act of thinking is related to an abstracted universal while maintaining its individuality. The third and final presupposition of Aquinas's theory is that form and matter (or soul and body in the case human beings) relate to each other in an asymmetric structure. For Aquinas, the intellective soul is ontologically prior because as the substantial form of the body, it gives being to the body. I will argue that this ontological priority allows Aquinas to defend the metaphysical possibility of identifying each individual human person as a genuine subject of thinking.


Aquinas's Critique of the Averroists. Aquinas's critique of Averroes's doctrine of intellect can be traced back to the very beginning of his career. In his commentary on the second book of the Sententiae written before 1256, Aquinas already argues that Averroes's notion of a separate intellect necessarily leads to the unacceptable result that a particular person such as Socrates does not think. (16) This accusation constitutes the core of Aquinas's attacks on Averroes's monopsychism in his later works, which reach their peak in the treatise specifically devoted to this controversy, De unitate intellectus contra Averroistas in 1270. (17) Since our concerns in this essay are more theoretical and systematic than historical, this section will focus on Aquinas's main arguments against Averroes and his followers in this short treatise, supplemented by parallel passages in Aquinas's other works. (18)

The Mechanisms of Thinking. Before entering into Aquinas's criticisms of Averroes, we need to have a rough idea of the mechanisms of thinking in Aquinas's epistemology. (19)

Above all, Aquinas holds that human cognition in this material world starts with the external senses. The process of sensation is a process of being impressed upon by the sensible properties of an external thing. (20) Following Aristotle, Aquinas interprets this process of being impressed as a special sort of reception in which a form is received without its matter, as in the case of wax receiving the imprint of a ring without its iron. (21) After being impressed, the wax obtains a shape similar to that of the ring. Accordingly, the sensitive soul receives a sensible form that is similar to that of a sensible object. Aquinas goes on to distinguish two kinds of impressions, one natural and the other spiritual. The precise meaning of this distinction is still an issue of controversy. (22) What is clear is that unlike a natural impression, a spiritual impression or change is the reception of a form F without becoming F-ed; for example, the eye's reception of red color without itself becoming red. According to Aquinas, in the case of sensation, a sensible form obtains an intentional or spiritual being (esse intentionale et spirituale) in the soul that is different from its being in a material object. (23) The sensible form in the soul is also called sensible species (species sensibilis), which somehow represents the sensible object. Nevertheless, a sense is a power in a bodily organ, and even the spiritual change necessary for sense perception takes place in the organ of sense. (24) Moreover, the process of being impressed also implies that there is something underlying the change as its subject. The intentional presence of a sensible species cannot come into being without the functioning of a bodily part. (25) More importantly for our purposes, a sensible species in the soul, though not in the sensible matter as an extramental sensible form, still maintains the individuating conditions of the matter. (26) This explains why the sensitive power can have cognition of only individual things.

When a sensible species occurs in the soul, internal senses, such as memory and imagination, will be activated to store and arrange the sensible species, together with their individuating conditions, in the mental images called phantasms. (27) Here, we touch the boundary between sensual and intellectual cognition in the Aristotelian tradition. Intellectual thinking is also a process of being impressed or being informed, but by a very different sort of forms, that is, intelligible species. In this regard, intellect is also a passive capacity of receiving forms and is therefore called the possible intellect, However, unlike sensible species, intelligible species are completely immaterial. Besides being in the cognitive powers of the soul, they are abstracted from both the individuating conditions of matter and the function of a bodily organ. (28) Aquinas insists that the intelligible species is the thing in virtue of which (id quo) we can think an object in an intellectual way, that is, in a way not limited by the constraints of matter and individuality, which will finally bring us to knowledge that is immaterial, universal, and necessary. (29) The intelligible species are so completely immaterial that they cannot exist in things that involve matter or individuate conditions of matter. That means even sensible species retained in phantasms are not intelligible unless abstracted from the individuating conditions. For sensible species are nothing but representations of sensible objects, which have material and individual existence. (30) other words, phantasms cannot directly impress their likeness on the possible intellect as colors do on our visual power, because the possible intellect can be impressed only by pure immaterial forms, which have a mode of existence entirely different from sensible species. (31) Therefore, to initiate a process of thinking, we have to posit an additional principle capable of abstracting the intelligible forms from phantasms. This active principle is called the agent intellect, in contrast with the possible intellect, which passively receives the forms. The abstractive mechanism of the intellect is still a matter of controversy, especially concerning whether the intelligible species is separated from phantasms, or whether it is instead generated as something new by the agent intellect. (32) matter what the agent intellect exactly contributes in the process of abstraction, what is clear is the sheer immateriality of the intellect as a cognitive power and the intelligible species as the result of abstraction. When the agent intellect abstracts an intelligible species from its material conditions and impresses it upon the possible intellect, the possible intellect then engages in acts of thinking such as forming a definition of the nature of an extramental object, which brings our cognition of an individual material thing to completion."

Considering his strong emphasis on the immateriality of thinking, it is surprising to find Aquinas taking the claim hic homo intelligit as the starting point of his philosophical arguments against Averroist monopsychism. (34) However, as noted earlier, Aquinas often asserts this proposition as an unquestionable premise without offering an argument. At one place, when he does offer an argument to defend it, he merely claims that it is a universal phenomenon that all can confirm by their experience of self-awareness: We all have the experience of being the one who thinks when we think. (35) Unlike Descartes, Aquinas does not think it necessary to question the certainty of this self-perception. (36) The reason is that this claim does not play a foundational role in Aquinas's metaphysical approach to the act of thinking, a point that will be clearer later in this essay. (37) Here it suffices to say that most medieval authors, even though they have different understandings of the contribution of human beings to thinking, agree that "this human being thinks" is a phenomenon that must be saved. (38)

The Doctrine of Turn Subjects. Aquinas presents two attempts of the Averroists to explain how a human being thinks when the principle of thinking is a separate substance, and then shows why they are unsuccessful. (39) Their first explanation is based upon Averroes' doctrine of two subjects. Their second one relies on a mover-moved model of the intellect and human beings.

According to the Averroists' first explanation, a single intelligible species has two subjects, the separate possible intellect itself and the phantasms found in human persons. In an act of thinking, the intelligible species unites us to the possible intellect through our phantasms. The possible intellect's act of thinking can be ascribed to us because the numerically same intelligible species informs both the possible intellect and our phantasms. (40)

Aquinas offers three objections to this Averroist explanation of how thinking can be said to be ours. First, he argues that the union of the separate intellect with human beings, as described by these Averroists, is not a natural or immanent union but simply an operational or functional combination, inasmuch as the separate intellect's act of thinking requires the operation of our sensitive power of imagination (fantasia). (41) In an earlier text, he stresses that the sort of union with the possible intellect that Averroes is proposing is merely a union on the level of action, namely, on the level of the Aristotelian second actuality, which is contingent for human beings. Since thinking is the sort of operation that distinguishes the human species from all other animals, however, he argues that Aristotelian psychology requires that we be united with the possible intellect immanently, on the level of first actuality, that is, the level of the soul as the substantial form of an animal. (42)

Aquinas's second objection concerns the numerical identity of intelligible species in two subjects. He argues that the possible intellect can receive an intelligible species only when it is in actuality. By contrast, a species in phantasms is merely intelligible in potentiality. (43) Our analysis of the mechanism of thinking has shown that we need the process of abstraction to make the species actually intelligible so that it can inform the possible object. As mentioned earlier, no matter how we interpret Aquinas's conception of abstraction, the intelligible species has a totally immaterial mode of existence different from species in our corporeal organs, since the latter still retains the individuating conditions of matter. Thus, phantasms and the possible intellect are informed by different kinds of species, and therefore have different acts of receiving forms, that is, different acts of cognizing. It is interesting that here, Aquinas returns to the sheer immateriality of thinking to deny the functional union between the separate intellect and our phantasms he conceded above for the sake of argument. Due to the inherent individuating conditions, it is not possible for phantasms to become a subject to which the intelligible species can inhere as Averroists propose. (44) Then Aquinas appeals to an analogy. He argues that the intelligible species in the possible intellect merely has a representational relation to phantasms, just as a person's appearance (species) reflected in a mirror has a representational relation to the person himself. However, the act of reflecting can be attributed only to the mirror and not to the person, just as the act of thinking can be attributed only to the possible intellect and not to our phantasms. (45)

Aquinas's third attack on the two-subjects doctrine seems to be most devastating. Conceding for the sake of argument that the form in the possible intellect is numerically the same with that in phantasms, Aquinas argues that the mere functional union between them is not sufficient to guarantee the attribution of thinking to the human being who has these phantasms. (46) His argument is based upon an analogy of the color on a wall. (47) According to Aquinas, when the color is seen, a sensible species representing the color occurs in the visual power of an animal. However, the functional union of the color on the wall with its sensible species does not make the color or the wall a thing that can see, because it does not have the visual capacity. Accordingly, the combination of the intelligible species with a person's phantasms cannot make the person a thing that thinks. For it is the cognitive power rather than the cognitive species that determines the attribution of a cognitive act. (48) What Aquinas has in mind here seems to be the Aristotelian principle that an action belongs to the thing to the which the power belongs. (49) It implies that a human being should have a power corresponding to the immaterial act of thinking to become its possessor. However, the genuine challenge for our approach is still how an immaterial power like intellect can be inherent to some extent in a material being.

The Mover-Moved, Model. In addition to advocating for the two-subjects theory, the Averroists claim that the intellect is somehow united with a material body (corpus) as its mover, which constitutes their second effort to save the phenomenon of Socrates seeming to think. Aquinas lists three possible versions of the mover-moved model: (1) Socrates is the whole mover-moved compound; (2) Socrates is merely the moved body, which is animated by the vegetative and sensual soul; (3) Socrates is just the possible intellect as the mover. (50)

Aquinas's objection to the first option relies primarily on an Aristotelian conception of substance: The mover and the moved cannot form a substance that instantiates a natural species, such as a horse. For Aristotle believes that only a union of form and matter (that is, actuality and potentiality) can make different parts a genuine whole other than a mere unintegrated aggregation of two different things. (51) If the Averroists conceded that the union of the possible intellect and the body is merely an accidental aggregation, it would follow that Socrates as the whole will not be a primary substance in Aristotelian categories. More importantly, it would therefore be impossible to attribute the action of a part to the mover-moved compound, for Aquinas assumes that the action of a part can be ascribed to the whole only when the whole in question is a genuine unity such as a primary substance. For instance, one cannot say that the thinking of a pilot belongs to the aggregate of the pilot and the boat moved by him. (52)

It seems even more ridiculous to think that the pilot's thinking can be attributed to the moved boat, as suggested by the second mover-moved model. First, Aquinas denies the possibility of such transition. He distinguishes transitive actions from intransitive ones. It is obvious that thinking is not an action that can be transferred to its object. (53) Second, even granting that the transition of thinking were possible, Aquinas insists nonetheless that the mover plays a more important role than the moved in determining the attribution of action. Therefore it is not appropriate to ascribe the act of thinking to its instrument such as Socrates. (54) Third, in a process of transition, what receives the effect of an act [PHI] is often said to be [PHI]-ed rather than [PHI]-ing. For instance, in the case of building a house, the art of building is transferred from the builder to the house. It is absurd to say that the house therefore can exercise the art of building. (55) Aquinas concedes that there is another form of transition, which allows a recipient to [PHI]. For example, when water is heated by fire, it can heat other things as well. Nevertheless, this is possible only when the water itself has heat as its form, in virtue of which it heats another thing. So if this is the case of Socrates' thinking, it follows that the principle of thinking must be a form of Socrates, which is precisely the position Aquinas himself take great pains to establish against the Averroists. (56) Throughout his anti-Averroist writings, Aquinas insists that only when the intellect is a power formally (formalitef) existing in us can its intellectual operation be ascribed to us. (57)

Before moving to Aquinas's own account of the individuality of thinking, we shall briefly mention his comments on the third option for the mover-moved model, according to which Socrates is identical with the mover, that is, with the possible intellect itself. Aquinas identifies this approach to the mover-moved model as Plato's view. In his evaluation of it, rather than dismissing it out of hand, Aquinas mentions its affinity to Aristotle's words that the element of intellect in a man can be thought to be the man himself. (58) Certainly, he reaffirms immediately the Aristotelian hylomorphism according to which Socrates is composed of soul and body. He lays a strong emphasis on the priority of the soul with respect to the definition of the body: "no part of the body can be defined without some part of the soul." (59) Without the soul, flesh and eye are called so only homonymously. (60) It is not difficult to see why Aquinas is not entirely hostile to the Plato's identification of a person with his intellect. For it at least offers an obvious explanation of how this human being thinks. What is problematic is the relation of this personal thinking to his corporeal being.

Aquinas's philosophical arguments against the Aveorrists present a more vivid picture of two seemingly incompatible themes in Aquinas's metaphysical approach to human thinking: the sheer immateriality of thinking on the one hand, and the corporeal being of a human person on the other. In particular, Aquinas's criticisms of the two-subject doctrine and the mover-moved model indicate that he thinks a more substantial relationship between thinking and person is needed. For Aquinas, only the relationship between form and matter can satisfy this requirement. He concludes that a person's act of thinking embodies a genuine unity of the possible intellect's function to the human person as a thinking thing. However, this is possible only when the possible intellect is a power of the soul that is united with us as the substantial form.61 Then he needs to explain how a thoroughly immaterial action can involve matter. In the section that follows, I will argue that the priority of the soul in Aquinas's hylomorphic anthropology suggests a way to incorporate these two aspects of his thought into a coherent account.


Aquinas's Positive Account for the Individuation of Thinking. In the last chapter of De unitate intellectus and other contexts, Aquinas takes great pains to tackle a series of problems relating to attributing the act of thinking to a human person. The following can help us better specify the theoretical challenges Aquinas has to face in maintaining that thinking is an act of a corporeal being.

Three Objections to Aquinas's Claim that "This Human Being Thinks." (1) The first and most important problem has to do with the ontological status of the intellect. If the possible intellect is not unified but multiplied according to the diversity of human beings, then one would assume that it must be individuated by the material distinction of human bodies. For the principle of individuation for a material compound is supposed to be the matter. Moreover, only a material form can be multiplied by the distinction of the matter. It necessarily follows that the possible intellect is nothing but a material form, which is in tension with Aristotle's belief that the intellect is something separate from matter. (62) Aquinas's opponents go even further to claim that a form separated from matter is neither numerically one nor something that can be individualized. (63) They also argue that, if the intellect were a material form, since all human bodies have a determinate nature, it would follow that the intellect would have a determinate nature in itself. This result is contrary to the nature of the possible intellect. The possible intellect has a natural capacity to know all things, and if the intellect had a determinate nature, this determinacy would impede its cognition of other things not sharing its determinate nature. (64) In short, the claim that "this human being thinks" seems to entail that the intellect is a material form, which is contrary to Aquinas's explicit assertion that the intellect is immaterial.

(2) The second objection to Aquinas's position concerns the intellect's status as a cognitive power. By definition, the possible intellect is the cognitive power of receiving intelligible forms. If the possible intellect is multiplied in the sense that my intellect is different from yours, then the intelligible form in my intellect will be different from the form in yours. For it is taken as an axiom that what is received is received according to the mode of the receiver. (65) For Aquinas's opponents, this axiom implies that the intelligible forms in our intellect are numerically distinct and individual forms. However, individual forms are intelligible only potentially, since a common intention or concept can be abstracted from them. For the same reason, this new intention is also individualized by our intellects, and then there will be another intention to be abstracted, which continues ad infinitum. In other words, the individuation of thinking will lead to the individuation of thought's object and ultimately make thinking impossible. (66)

(3) The last objection relates to Aquinas's commitment to the immortality of intellective souls and touches one of the most difficult challenges to his ontology of thinking. The objection is based upon a fundamental principle of causation: When the cause is taken away, so too is the effect. If possible intellects and their acts of thinking are multiplied in accordance with bodies (secundum corpora), then they will not remain when the bodies have been destroyed, and the Christian belief in postmortem rewards and punishments will lose its ontological ground. (67)

Aquinas on the Ontological Status of the Intellect. In Aquinas's initial response to the first objection in his commentary on the Sentences of Peter Lombard, it is somewhat surprising to find that he does not think it is contradictory to assert that the intellect is a material form.
   [I]t should be said that the intellect is not denied to be a
   material form so that it might be prevented from giving being to
   matter (quin (let, esse materiae) as a substantial form, with
   respect to its first being. For this reason, it is necessary that
   the multiplication of the intellect, that is, of the intellective
   soul, follow upon the division of matter which causes diverse
   individuals. But it is called immaterial with respect to its second
   actuality, which is an operation; because thinking does not take
   place by means of a mediating bodily organ. This occurs because an
   operation proceeds from the essence of the soul only through its
   mediating power or potency. Hence, since it has some powers which
   are not acts of certain organs of the body, it is necessary that
   certain operations of the soul do not occur through a mediating
   body. (68)

It seems that the young Aquinas adopts a materialist view of the intellect, probably for its terse simplicity in clarifying the individuation of intellect. At this point in his intellectual development, when he holds that the intellect is a material form, Aquinas explains the diversity of the intellect--as with the diversity of other material being--in terms of the division of matter. Matter remains the principle of individuation. Socrates' intellect is distinct from Plato's primarily because they inform two different bodies. Then Aquinas appeals to the ambiguity of the Latin word intellectus to show how this materialist view can be compatible with the immateriality of intellect. The Latin word intellectus can refer to either the intellective soul as the substantial form of the body or the intellective power that serves as the immediate principle of thinking. For Aquinas, the possible intellect denotes a cognitive power of the soul. However, in the passage cited above he is answering an argument that claims, "the rational soul or intellect is one in number in all human beings.'" (69) This explains why these two terms are used interchangeably in Aquinas's response. It follows that Aquinas merely claims that the intellective soul is multiplied as a material form. Aquinas also insists that the intellective soul cannot be the immediate basis of its operations but rather operates through the mediation of its intellective powers. (70) Only when taken as a power of the soul, the intellect is immaterial in that its operation, namely thinking, does not involve any bodily activity. Here, by implicitly invoking his controversial doctrine of a real distinction between the essence of the soul and its powers, (71) Aquinas concludes that one cannot directly infer from the immateriality of one of the soul's powers that the soul itself is also immaterial.

Aquinas's early account, however, has significant defects. Above all, it risks seeming incoherent. He explicitly denies that the rational soul is a material form when he talks about the origin of the human soul in the same work, which immediately follows the one cited above: "The rational soul is neither composed of matter, nor is a material form, as if [it is] merged in the matter." (72) If we do not believe that Aquinas could be making contradictory claims, we must pay careful attention to the qualifications he makes in these two claims. In the earlier text, he identifies the intellect or the rational soul as a material form merely in that it "gives being to matter as a substantial form." In contrast, when he denies that the rational soul of a human being is a material form that can be generated by his parents, he means that the soul is not wholly limited by its matter in such a way that its operations cannot be without the body. In short, the intellective soul is not a material form without qualification, but a special one that can have both material individuation and immaterial capacities. Nevertheless, if we do not want to take this as a merely ad hoc explanation, we still need to clarify how this combination of two characteristics of the soul is metaphysically possible. In particular, we need to elucidate in what sense the intellective soul gives being to the matter, an obscure but important claim we shall return at the end of this essay.

Furthermore, the distinction between the intellective soul and the intellective power is a distinction between substantial and accidental forms. For Aquinas takes the powers of the soul as accidental forms, though as necessary ones flowing out from the essence of the soul. (73) As mentioned above, Aquinas recognizes a real distinction between the soul and its powers. Taking this for granted, one may still wonder how an immaterial power can belong to a material form. This seems to make an accidental form prior to its substantial form in dignity or ontological status, which conflicts with Aquinas's ontology of priority and at least asks for further explanation. This point turns out to be the foundation of an objection Aquinas has to deal with later in the Summa Theologiae. (74) However, his response merely repeats the aforementioned ad hoc solution that the human soul is a form of the body (corporis forma) that is not totally merged in the corporeal matter and therefore can have some incorporeal power like the intellect. (75) It is noteworthy that here Aquinas no longer refers to the intellect or the intellective soul as a material form, but rather as a "form of the body." For Aquinas unambiguously claims in the Summa that material formality is incompatible with the subsistence of the human soul. (76) Moreover, Aquinas's response to the objection seems to presuppose that a form not totally merged in the matter, whether it is called a material form or form of the body, is not posterior to the immaterial power of thinking in dignity or in ontological status. It seems that the intellective soul is at least as immaterial, simple, and abstract as the intellective power of thinking, for it is also not constrained by the capacities of the matter. (77) Therefore, one cannot claim without additional argument that the intellective soul as such is nonetheless a form of the body rather than an immaterial entity.

On the other hand, by abandoning his view of the intellect as a specific material form, Aquinas also has to give up matter as the principle of the individuation of thinking. As I will argue, he actually mentions something more fundamental to account for the individuality of both material and immaterial beings. To better understand this approach, we need to clarify in advance a few significant conceptual distinctions concerning individuality.

First, individuality cannot be identified with multiplicity, especially not with the multiplicity of instances within a species. A thing can be individuated without becoming multiplied. For God is an individual, but there is only one God. (78) Further, for Aquinas, each species of angel only has one individual. Angels are individuated by their intrinsic natures but cannot be numerically multiplied by anything. (79)

Moreover, individuality is different from unity as well. For unity is said in many ways: numerical, specific, general, and proportional. (80) For instance, two human beings can be one or identical in species in that they have the same definition. It is obvious that only numerical unity is concerned with the individuality under consideration. Aquinas emphasizes as well that the other sorts of unity are not unity without qualification (simpliciter). (81)

Furthermore, even numerical unity cannot be identified with individuality. Aquinas makes a significant clarification on their relationship: A thing is called numerically one (unum numero) not because it is one in number (unum de numero), but because it is not divided when being numbered. The indivisibility is the cause of numerability, not the other way around. (82) In other words, numerical unity is a phenomenon that needs to be accounted for by a more profound ontological indivisibility. Returning to the case of thinking, we can say that any act of thinking as an instance of action is necessarily an action that is numerically one. However, this says nothing about the principle of its individuation, for even a separate intellect can have such a single action. It can even be multiplied according to the difference of its objects. My thinking of a mathematical object is numerically different from my understanding of the metaphysical principle of individuation. The fact that it is an instance of action does not explain why it should be ascribed to a particular individual that is acting. For that purpose, we need to show the essential link between the act of thinking and the individuality of its agent.

Finally, we should distinguish the problem of individuality of thinking from the question of personal identity over time. For the metaphysical possibility of individual thinking is primarily concerned with the synchronic unity between a material human being and his immaterial act of thinking, while personal identity is more concerned with the diaehronic continuity of this union.

Therefore, what concerns us is the fundamental unity or indivisibility of an individual person at any given moment that grounds our attribution of the act of thinking to him. This act is individualized not only because it is a numerically single act, but more importantly because it belongs to a person who is unique and irreplaceable. As Aquinas insists, in created things, the individuating principle should explain not only their subsistence but also the difference of those who share a common nature. (83) Only with such a conception of individuality is it possible for us to distinguish Socrates' thinking from Plato's.

Now we return to Aquinas's response to the first objection mentioned above in the De unitate intellectus. Above all, Aquinas argues that God and angels as separate substances are necessarily individual and singular (indiuidue et singulares), because otherwise they cannot have any operation or action. (84) As Alain de Libera rightly observes, Aquinas's position on this point is based upon a misunderstanding of Aristotle's famous claim that actions are only concerned with individuals as objects (actus sint solum singularium) to mean that only individuals as subjects can act, (85) It follows that God and angels act only as individuals even though they are separate from matter and the material world. Aquinas goes further to claim that even Platonic ideas are individuals in this sense and therefore cannot be defined or predicated of many things. In light of this understanding of immaterial individuality, Aquinas reinterprets the Aristotelian doctrine that matter is the principle of individuation. He introduces a new conception of individuality that he sees as applying to both material and immaterial realities:
   Matter is the principle of individuation in material things insofar
   as matter is not participated in (participabilis) by many, since it
   is the first subject not existing in another.... Separate
   substances, therefore, are individual and singular, but they are
   individuated not by matter but by the fact that they are not
   destined to (nate) be in another thing and consequently to be
   participated in by many. From which it follows that if any form is
   destined to be participated in by another, so that it become the
   actuality of some matter, then it can be individuated and
   multiplied by its combination (comparatio) with matter. (86)

It is clear now that a thing is called individual because it cannot be further participated in by any other subject-substratum. In other words, imparticipability and individuality are equivalent in this context. It is also evident now that Aquinas does not recognize two principles of individuation in human beings as some commentators wrongly suggested: "like material things, we are individuated by matter. Like immaterial things, we are individuated by our intellectuality." (87) For imparticipability univocally explains the individuality of both material and immaterial things.

This conception of imparticipability offers an alternative answer to the individuality of thinking that replaces the conception of intellect as a special material form. The summary above of Aquinas's critique of Averroists' doctrine of two subjects already indicates that, for Aquinas, thinking is ascribed to a person because he has a cognitive power that serves as the active principle of thinking. Now Aquinas goes on to confirm that cognitive powers are individuated in virtue of the substantial form to which they belong, that is, the intellective soul. The soul itself is further individuated by its essential relation to the body. (88) This is not a simple return to the matter as the principle of individuation, because material individuality can be explained in terms of imparticipability as well. Nevertheless, unlike separate substances that are destined or made according to their nature (natus est) to be imparticipable, the ultimate source of the imparticipability of a composite substance is the primacy of matter as subject-substratum. However, from very early on, Aquinas is quite clear that matter cannot be called a subject-substratum in its strict sense because it does not have a complete being like a subject-substratum of an accident. (89) Strictly speaking, the first subject that cannot be further participated in by other things is the compound of form and matter. In the case of human beings being the agents of their thinking, we are brought back to the question about the ontological compatibility between the intellective soul and the material body. Before returning to this fundamental issue, we have a more urgent question. Granting that our intellective power is individuated because our intellective souls are individuated, it is still unclear how this individual power functions in our thinking. For Aristotle claims that thinking is immaterial in that it is not the actuality of matter. However, in Aquinas's opponents' eyes, the individuality of the intellective power seems to conflict with this claim.

Aquinas on Intelligible Species. Aquinas's response to the second objection relies on his peculiar conception of intelligible species, which is another topic of ongoing interpretative controversy. (90) Since our concerns are more ontological than epistemological, we will simply focus on its role in determining the individuality of thinking. As mentioned above, a most significant point for Aquinas's species theory of cognition is that a species is the thing in virtue of which (id quo) an intellect can think or understand an intelligible form in its actuality. Aquinas insists that within the Aristotle's philosophical framework, what is understood (intellectum) is normally not an intelligible species but the nature or quiddity of a thing, which remains the same for all intellects. (91) This sharp distinction between intelligible species and the object of thinking is central to Aquinas's critique of Averroes and his followers. (92) Whether the intelligible species is formally identical with the extramental nature or just its mental representation is still a controversial issue. (93) What is unquestionable is that the species plays two different roles in the process of thinking. On the one hand, the intelligible species is the form or intention that the agent intellect abstracts from phantasms and is received into the possible intellect. According to the axiom that what is received is received in the manner of its recipient, the intelligible species is an accidental form received in the possible intellect in accordance with the ontological status of the possible intellect. Since Aquinas believes that the possible intellect is a power of an individual soul, it follows that the intelligible species is individuated in the intellect. On the other hand, the intelligible species is abstracted from material and individuating conditions of phantasms. It is an intention in the mind representing the extramental thing "not in its individual conditions, but only according to the universal nature." (94) This is the so-called theory of the double being of the intelligible species. According to this theory, the intelligible species has both an individual ontological being and a universal intentional being. (95) Thanks to the double being of the intelligible species, (96) Aquinas proposes a solution to the puzzles revolving around the universality and individuality of thinking:
   Therefore, there is one thing that is thought by me and by you, but
   it is thought in virtue of one thing by me and in virtue of another
   by you, that is, by different intelligible species, and my thinking
   is different from yours and my intellect is different from yours.

My thinking is distinct from yours because it is essentially rooted in my individual power of cognition. Nevertheless, our thoughts can be communicated because they are related to the same object, either extramental or representational, which has been abstracted from its material and individuating conditions. In most cases, the object of thought is the universal nature of things. However, there is still a marginal issue that needs to be addressed here. Aquinas mentions that in some special cases, when the intellect reflects on itself, the intelligible species received in the possible intellect will become the object of intellectual thinking. (98) Since the intelligible species in the intellect is an individual form, Aquinas still needs to explain how this individual form can be understood as something intelligible in actuality. Aquinas's answer is rather simple: What is incompatible with thinking is not singularity but materiality. Since intelligible species are immaterial individuals just like separate substances, which obviously can each individually think about themselves, nothing prevents the individuated intelligible species from being thought in the intellect's self-reflection. (99)

Aquinas's incorporation of intelligible species into his account of the individuality of thinking represents significant progress. For its double aspect helps us better conceive of the combination of individuality and immateriality in a single entity. It offers a mechanism to show how the act of thinking is related to a universal while maintaining its individuality. An intelligible species is individuated because it is an intentional being that is retained in the possible intellect. This gives us a good reason to ascribe the act of thinking to the intellect in virtue of this individuated species. However, can we therefore ascribe thinking to the person who has the intellect? Aquinas seems to imply so. He mentions that the human intellect has a special need of intelligible species in order to know, because it has no immanent knowledge at all. A human intellect needs sensation and imagination to obtain its own intelligible species. This understanding of the human intellect seems to indicate that our corporeal existence contributes to the acts of our intellects. However, it merely seems so. For as is shown in Aquinas's critique of Averroists' theory of two subjects, the contribution of phantasms to the process of thinking is something that needs to be deprived of its individuating conditions. Therefore, even if we can accept the primitive individuality of intellect and its own intelligible species, what we have achieved is merely to ascribe the act of thinking to the intellect, not the person.

Aquinas on the Intellect and the Body. Aquinas's response to the third objection, the one concerning the soul's individuality after death, strengthens the aforementioned tendency of his thought to link the act of thinking to an individual intellect rather than to an individual person. With the new principle of individuality at hand, Aquinas gives a simple answer to the objection about the individuality of the intellect that survives death: The unity of a thing depends upon its being. Since the intellect (and the intellective soul) has its own being, it will not be taken away when the body is destroyed. Therefore, the intellect's unity and the individuality flowing from its existence will remain after death. (100)

This argument obviously presupposes the immortality of the soul, which is not put into question by the Averroists Aquinas has in mind. Nevertheless, a crucial point in Aquinas's argument for the immortality of the soul is that the intellective soul has its own operation, namely thinking, which is not shared by the body. This is so because the intellect by nature can cognize the natures of all bodies. As shown in the first objection above, medieval Aristotelians believed that if the intellective soul had a determinate material nature, this would impede its cognition of other things. Therefore, it is impossible for the intellective soul as the principle of thinking to be a body or to operate through a bodily organ. It follows that the intellective soul has thinking per se that the body does not share in. (101)

Whether this argument for the subsistence of the intellect is valid is not our concern here. What troubles us here is the argument's explicit claim that thinking is an operation of the intellective soul itself, because this claim seems to be incompatible with his hylomorphic conception of human being. As mentioned above, Aquinas misinterprets Aristotle's claim that actions are concerned only with individual objects to mean that actions merely belong to individual subjects. (102) It is well known that Aquinas's ontology adopts an even stronger version of this principle: Actions belong only to supposita (actiones sunt suppositorum). (103) Since a suppositum is understood as an individual that subsists in the genus of substance, (104) it follows that strictly speaking only a human person can qualify as the agent of his thinking. Aquinas himself also unequivocally acknowledges this point in his argument for the immortality of the soul: "one can say that the soul thinks, just as the eye sees. But one speaks more strictly in saying that the human being thinks through the soul." (105) So, to repeat again our puzzle from the beginning: How can the same action of thinking be ascribed both to the intellect and to the human person who is thinking?

It has been suggested that Aquinas proposes a straightforward way to link thinking, intellect, and the human person by his formal conception of the intellect: The (possible) intellect is the thing, formally speaking (formaliter loquendo), in virtue of which a human being thinks. (106) Aquinas here is alluding to the principle of intrinsic formal cause, according to which a thing acts only when the principle of action is its intrinsic form. (107) It is evident by definition that the intellect is the principle of intellectual thinking. However, one should be careful directly to draw the conclusion that the intellective principle of human thinking is a form of the human body, as Aquinas implies here. (108) In his treatise against the Averroists, Aquinas cautiously elucidates that the intellect formally inheres in a human person, "not in that it is the form of the body, but in that it is a power of the soul that is the form of the body. (109) This is so because Aquinas believes that an operation or action is ascribed to an agent only by means of a power that is the immediate or proximate principle of the action. (110) It means that the intellective soul can be called the nonproximate principle of thinking only in a derivative sense. Furthermore, by attributing the act of thinking to the intellective soul via the intellective power, we are presupposing that the possible intellect can be identified as a power of the intellective soul. And as we argued above, this is possible only when the intellective soul itself is held to be as immaterial as its intellective power. (111) For even Averroes can acknowledge that the possible intellect is a power of the intellective soul, provided that the soul is a completely separate substance. (112) Averroes' idea turns out to be similar to Aquinas's argument concerning the individuality of intelligible species. However, for Aquinas, the presence of the intelligible species in the intellective soul merely shows that the soul thinks; it does not show that the person thinks. The greatest challenge for Aquinas's theory of individual thinking is still how an immaterial soul can be united with the body as its substantial form while the act of thinking remains an operation of the soul per se. Does Aquinas have a way out of the dilemma we have been struggling with?

I think we need to return to a general point of Aquinas's hylomorphism we mentioned earlier, namely, the priority of form over matter. I will mention here only briefly a beautiful expression of this priority found in Aquinas's early account of intellect: "form gives being to matter (forma dat esse material. (113)

Some cautions should be taken into consideration before we apply this terse expression to the special case of human being. Above all, in this phrase, form is simply a shorthand for substantial form. Likewise, as I have argued elsewhere, the matter in question is nothing but prime matter in the sense of pure potentiality. (114) Aquinas maintains that it is the form that provides or completes the being of the matter by making it actual. (115) When Aquinas insists that form gives being to matter, it should not be understood as if there are two separate entities that exist on their own and then one of them bestows the act of being on the other. Before obtaining a form or the form, the matter, absolutely speaking, does not exist, On the other hand, a material form normally cannot exist without the matter. It follows that the hylomorphic compound, not the material form, is the genuine possessor of the being given by the form. In other words, the verb "to give" signifies a special sort of ontological priority of form to matter, which does not have an existential connotation. It does not imply that form can exist without matter and then brings matter into being. It merely means that the form plays a dominant role in explaining the act of being of a hylomorphic compound.

In the case of human beings, one can easily infer that the being of the human person exclusively originates from the intellective soul as his substantial form. Aquinas rejects the idea that there are other substantial forms such as corporeity, vegetative soul, animal soul, and so on that also determine the being of a human person. The intellective soul therefore determines the whole person's mode of being as its sole substantial form. (116) So when the essence of a person is concerned, we can say that we are essentially our intellective souls.

In his arguments against the Averroists, Aquinas also mentions ancient commentator Themistius's distinction between "I" and my essence, which can be used to support the above startling claim. (117) According to Themistius, "I" as a thing that really exists in this material world is composed of something in actuality and something in potentiality. However, my essence is defined by what I actually am. Themistius argues that my essence can come only from the soul that is the actuality of the body, and not from the vegetative and sensitive soul, because they are matter for the intellectual power. (118) No doubt, Aquinas cannot accept an unqualified identity of a human person with his intellective soul. As mentioned earlier, this is taken as a Platonic position he openly rejects because it fails to explain how the same human person can be the subject both of his thinking and of his sensation. (119) However, this does not mean that Aquinas cannot accept their identity in regard to being. For the soul gives being to the body and therefore shares the same act of being with the human person of whom it is the substantial form.

In regard to the ontological priority of the soul in determining the being of a human person, we can say that there is a reduction in Aquinas's hylomorphic ontology that is inverse to materialism. It is not that the soul's being should be explained in terms of the body's being, but the other way around. In this light, it does not matter what the body contributes to the act of thinking as a constituent of the human person. For whatever it contributes, it contributes in virtue of the intellective soul as its unique substantial form. Only when the intellective soul is present can the matter of a human being be a body. In this sense we can say that it is the intellective soul that is thinking, even though strictly speaking it is not the agent of thinking. For the agent of thinking in this world is a living person whose being originates exclusively from the intellective soul. The fact that thinking is the intellective soul's own operation does not threaten the substantial unity required by Aquinas's hylomorphic anthropology. This is possible because he accepts the principle that a thing's unity also originates from its substantial form. For instance, in the Summa contra Gentiles Aquinas applies this principle to argue for the unity of substantial form in the human being:
   Moreover the principle of a thing's unity is the same as that of
   its being; for one is consequent upon being. Therefore, since each
   and every thing has being from its form, it will also have unity
   from its form. Consequently, if several souls, as so many distinct
   forms, are ascribed to man, he will not be one being, but several.

Therefore, if the intellective soul gives being to the human person, it also determines its substantial unity. This claim about the convertibility of unity and being is a general one that can be applied to all hylomorphic compounds. It follows that even material forms are prior to their matter in determining the being and unity of the whole compound. With this general picture of hylomorphic unity in mind, the aforementioned ambiguous status of the intellective soul as a form of the body that is not totally merged in the matter is no longer an ad hoc explanation as it seemed to be prima facie. For this special status of the intellective soul goes along with the ontological priority of form to matter. What is changed here is the existential connotation of this priority: Now the intellective soul can exist without the body, since Aquinas believes that there will be a separated soul between human death and the general resurrection. (121) Certainly, much work still needs to be done to justify the introduction of the soul's existential priority. Nevertheless, Aquinas's commitment to the ontological priority of form as reconstructed above at least indicates a way to explain how the substantial form of a material compound can perform an immaterial operation.

Returning to the dilemma we have been struggling with, we are now in a position to say that Aquinas can indeed maintain the immateriality of thinking without destroying the natural unity of the thinking person or abandoning his hylomorphic conception of the human person. For the ultimate principle of thinking and the principle of unity is precisely the same intellective soul. It is metaphysically possible for this immaterial soul to be the form of the body simply because the ontological priority of form allows form (in the case of the immaterial soul) to have a mode of being that is not fully shared or consumed by the matter. This metaphysical possibility explains why Aquinas claims in his early works that the intellective soul is an absolute form (forma absoluta) that has its own absolute being (esse abolutum) independent of matter. (122) Whether this earlier notion of absolute form and his later conception of subsistent substantial form go beyond the boundary of Aristotelian hylomorphism turns on how we understand the ontological privilege of form in his ontology. (123) At least one can say that some Aristotelian scholars still think that this priority of form, especially in the case of the human soul, is an essential characteristic of his philosophical psychology. (124)

Nevertheless, there is still a problem about the subject-agent of thinking. If the intellective soul has thinking as its own operation, how can we be said to be the agents of our thinking? Does the same action of thinking have two different subject-agents at the same time?

The last question presumes that the intellective soul should be an independent subject-agent to have its own operation. Therefore, to solve this problem, we need to elucidate what is required for a thing to have an operation on its own. Aquinas claims that only a thing that subsists on its own can have such an operation. (125) However, he immediately makes it clear that the subsistence in question should be understood in a weak sense. This means that a thing subsists on its own when it is not an accident or a material form that inheres in another thing, even if it is a part. (126) Therefore, to have an operation on its own means simply that there is a subsistent thing that is the sole source of its operation. It does not matter whether the thing has a complete being like a suppositum or whether something exists as an essential part in a suppositum. Applying this to the case of thinking, we can infer that thinking ultimately originates from the intellective soul alone. It does not necessarily follow that the soul should be either its immediate principle or its agent. For the immediate principle is the intellective power of the soul, while the agent should be a suppositum possessing a complete being, either the composite of the soul and body in this life, or the separate soul after death but before resurrection, or the composite of the soul and the resurrected body after resurrection. For the soul to have an operation as its own only requires that the soul is the sole source of being in all cases. Certainly it will follow that the soul has different modes of being, and how to establish the personal identity in these modes will be a great challenge for a Christian believer in the immortality of the soul. Here I want only to mention that in all these occasions the synchronic individuality of the soul resides in its inherent imparticipability, as we argued earlier. If we can accept that this imparticipability of the soul survives death, we can find a way to account for the sameness of individuality in the soul. Since Aquinas believes that the human soul is directly created by God, he has good reason to believe that this individuality directly coming from God will not evaporate after death. We need more studies to establish this point adequately, however. (127) What is more relevant to our present purposes is that the intellective soul can possess the same power of thinking by giving being to different things, be it a corruptible body in this world, or an incorruptible body in the life to come, or something else in between.


Conclusion. Aquinas maintains an intimate relation between the act of thinking and the human person as a material being, which manifests itself in his dictum "this human being thinks." Accordingly, he believes that a functional conjunction of a separate intellect with a living body cannot provide a basis for ascribing the immaterial action of thinking to individual human beings. For Aquinas, the act of thinking can be described as an individual action, first of all, because it requires the mediation of the intelligible species. However, the intelligible species has two different perspectives: it is both an intention representing the nature of things and an accidental form that exists in the possible intellect. In regard to its intentional being, the intelligible species is an immaterial representation of the extramental nature in that it has been abstracted from all the individuating conditions of that nature. From the perspective of its existence in one's possible intellect, the intelligible species is individuated in accordance with the being of the possible intellect. However, the possible intellect is merely an incorporeal power of the intellective soul. It follows that the individuality of the possible intellect comes from the individuality of the intellective soul. However, the intellective soul itself is individualized because of its inherent imparticipability, not because of its presence in a material body. On the contrary, the soul as the substantial form determines the being of a human person in this life. If the intellective soul can have another sort of being without the body, its act of thinking can still be individualized because of its immanent individuality. On the other hand, thinking is an immaterial operation because its immediate principle, the intellective power, is not mixed with the matter. However, this power is an accidental form flowing from the essence of the soul. As we have argued, this implies that the intellective soul is the ultimate source of thinking's immateriality. The immateriality of thinking now no longer conflicts with its individuality, because they both originate from the intellective soul as the substantial form of human beings. We therefore have good reason to say that the intellective soul is the ultimate principle of thinking, whereas in this life only a human person who receives his being from the intellective soul is the genuine subject-agent of thinking. (128)

Peking University

Correspondence to: Department of Philosophy, Peking University, 100871 Beijing, China.

(1) Alain de Libera, Archeologie du sujet I: Naissance du sujet (Paris: Vrin, 2007), 15-30.

(2) Alain de Libera, L'unite de I'intellect: Commentaire du De unitate intellectus contra averroistas de Thomas d'Aquin (Paris: Vrin, 2004), esp. 9-11; Alain de Libera, Archeologie du sujet I, esp. 52-59, 303-11; Alain de Libera, "When Did the Modern Subject Emerge," American Catholic Philosophical Quarterly 82, no. 2 (2008): 181-220, at 210-11; Alain de Libera, Archeologie du sujet II: La quete de I'identite (Paris: Vrin, 2008), 136-39; Alain de Libera, Archeologie du sujet III.1: L'acte de penser: la double revolution (Paris: Vrin, 2014), 245-56.

(3) For the difficulties of rendering the Latin term intelligere into English, see Anthony Kenny, Aquinas on Mind (London: Routledge, 1994), 41-42. It refers primarily to rational cognition of a thing's universal properties, through which a knower obtains necessary information for knowledge. It covers both dispositional apprehension as well as occurent acts of thinking.

(4) Averroes, Commentarium magnum in Aristotelis De anima libros, ed. F. S. Crawford (Cambridge, Mass.: The Medieval Academy of America, 1953), 3.5.

(5) Aristotle, De anima, ed. W. D. Ross (Oxford: Clarendon Press, 1961), 3.4.429a24.

(6) Since the fact that a person thinks is above all confirmed by this person's awareness of his thinking, Aquinas's reflections on self-knowledge in this regard have received considerable attentions in recent scholarship. See in particular Francois-Xavier Putallaz, Le sens de la reflexion chez Thomas d'Aquin (Paris: Vrin, 1991); Deborah L. Black, "Consciousness and Self-Knowledge in Aquinas's Critique of Averroes's Psychology," Journal of the Histor-y of Philosophy 31, no. 3 (1993): 349-85; and Therese Scarpelli Cory, Aquinas on Human Self-Knowledge (New York: Cambridge University Press, 2014). However, unlike Avicenna, Aquinas does not identify self-awareness as the foundation for the individuation of human intellect as an immaterial being. For a brilliant study of Avicenna's theory of self-awareness, see Jari Kaukua, Self-Aivareness in Islamic Philosophy (Cambridge: Cambridge University Press, 2015), esp. 43-61.

(7) See, for instance, Siger of Brabant, De anima intellectia, in Quaestiones in tertium de anima, De anima intellectiva, De aeternitate mundi, ed. B. C. Bazan (Louvain: Peeters, 1972), p. 84, 11. 49-51: "Thomas etiam intentum non arguit, sed solum quaerit eius ratio quomodo compositum materiale intelligeret, ut homo, si anima intellectiva in essendo sit separata a materia et corpore." Anonymous, Quaestiones in Aristotelis libros I et II De anima, ed. Maurice Giele, in Trois commentaires anonymes sur le Traite de I'ame d'Aristote (Louvain: Publications universitaires-Nauwelaerts), bk. 2, chap. 4, p. 75: "Isti autem accipiunt quo homo proprie intelligit, nec hoc probant. Ex hoc supposito arguunt. Quodsi istud suppositum non est verum, non arguunt." See also St. Thomas Aquinas, Summa Theologiae (hereafter, ST), Opera Omnia, Leonine ed. (Rome: Typographia Polyglotta S. C. de Propaganda Fide, 1888-89), I, q. 76, a. 1: "Si quis autem velit dicere animam intellectivam non esse corporis formam, oportet quod inveniat modum quo ista actio quae est intelligere, sit huius hominis actio, experitur enim unusquisque seipsum esse qui intelligit." For recent research on other medieval authors' criticisms of Aquinas's position on hie homo intelligit, see Concetta Luna, "Quelques precisions chronologiques a propos de la controverse sur l'unite de l'intellect," Revue des Sciences Philosophique et Theologiques 83 (1999): 649-84; Brian Francis Conolly, "Averroes, Thomas Aquinas and Giles of Rome on How This Man Understands," Vivarium 45, no. 1 (2007): 69-92; Cecillia Trifogli, "Giles of Rome against Thomas Aquinas on the Subject of Thinking and the Status of the Human Soul," Documenti e studi sulla tradizione filosofica medieval 23 (2012): 221-44; Marilyn McCord Adams and Cecilia Trifogli, "Whose Thought Is It? The Soul and the Subject of Action in Some Thirteenth and Fourteenth Century Aristotelians," Philosophy and Phenomenological Research 85, no. 3 (2012): 624-47; Jean-Baptiste Brenet, "Sujet, objet, pensee personnelle: l'Anonyme de Giele contre Thomas d'Aquin," Archives d'Histoire Doctrinale et Litteraire du Moyen Age 79 (2012): 49-69.

(8) See, for instance, Bernardo-Carlos Bazan, "The Human Soul: Form and Substance? Thomas Aquinas' Critique of Eclectic Aristotelianism," Archives d'Histoire Doctrinale et Litteraire du Moyen Age 64 (1997): 95-126, Bernardo-Carlos Bazan, "The Creation of the Soul according to Thomas Aquinas," in Philosophy and Theology in the Long Middle Ages: A Tribu te to Stephen F. Brown, ed. Kent Emery, Jr. et al. (Leiden: Brill, 2011), 515-69; Jean-Baptiste Brenet, "' ... set hominem anima': Thomas d'Aquin et la pensee personnelle comme action du 'compose'," Melanges de l'Universite Saint Joseph (Beirut) 59 (2006): 69-96, Jean-Baptiste Brenet, "Thomas d'Aquin pense-t-il? Retours sur Hie homo intelligit," Revue des Sciences Philosophique et Theologiques 93, no. 2 (2009): 229-50; Antonio Petagine, Matiere, corps, esprit: La notion de sujet dans la philosophic de Thomas d'Aquin (Fribourg: Academic Press Fribourg Suisse, 2014), esp. 218-29.

(9) De Libera, Archeologie du sujet I, 303-11; de Libera, Archeologie du sujet III, 1, 245-52.

(10) De Libera, Archeologie du sujet III, I, 352-53; 377-95.

(11) An interesting case is Brenet. He first claimed that Aquinas's hic homo intelligit is a self-evident claim as the law of noncontradiction that cannot be denied. See Brenet, "' ... set hominem anima'," 70 n. 4. Then he took it seriously and set to examine in a later paper whether Aquinas himself is able to justify this claim from a theoretical point of view. See Brenet, "Thomas d'Aquin pense-t-il?" 229. However, Brenet is still more interested in the problem of attribution than the problem of individuality.

(12) See, for instance, Gyula Klima, "Aquinas on the Materiality of the Human Soul and the Immateriality of the Human Intellect," Philosophical Investigations 32 (2009): 163-82; Brenet, "Thomas d'Aquin pense-t-il?" esp. 241.

(13) See Adams and Trifogli, "Whose Thought Is It?" esp. 631.

(14) See, for instance, de Libera, "When Did the Modem Subject Emerge," 210-11; Richard Cross, "Accidents, Substantial Forms, and Causal Powers in the Late Thirteenth Century: Some Reflections on the Axiom Actiones sunt suppositorum'," in Complements de substance: Etudes sur les proprietes accidentelles offertes a Alain de Libera, ed. Christophe Erismann and Alexandrine Schniewind (Paris: Vrin, 2008), 133-46.

(15) The failure to appreciate this point leads some commentators wrongly to claim that, Aquinas has to accept the intellective soul as a material being. See, for instance, Conolly, "Averroes, Thomas Aquinas, and Giles of Rome," 72.

(16) St. Thomas Aquinas, Scriptum super libros Sententiarum magistri Petri Lombardi episcopi Parisiensis (hereafter, In Sent), Tomus 2, ed. Pierre Mandonnet (Paris: P. Lethielleux, 1929), bk. 2, d. 17, q. 2, a. 1: "Si ergo non conjungitur intellectus nobiscum, nisi per hoc quod species intellecta aliquo modo habet subjectum in nobis, sequitur quod hie homo, scilicet Socrates, non intelligat, sed quod intellectus separatus intelligat ea quae ipse imaginatur." For comments on Aquinas's first effort to deal with Averroes' monopsychism, see Richard Taylor, "Aquinas and 'the Arabs': Aquinas's First Critical Encounter with the Doctrines of Avicenna and Averroes on the Intellect, In 2 Sent. d. 17, q. 2, a. 1," in Philosophical Psychology in Arabic Thought and the Latin Aristotelianism of the 13th Century, ed. Luis Xavier Lopez-Farjeat and Jorg Alejandro Tellkamp (Paris: Vrin, 2013), 141-83.

(17) St. Thomas Aquinas, De unitate contra Averroistas (hereafter, DUI), in Opera Omnia iussu Leonis XIII P. M. edita, Tomus 43 (Rome: Editori di san Tommaso, 1976), 289-314. For the chronology of Aquinas's works, I follow Jean-Pierre Torrell, Initiation a saint Thomas d'Aquin: Sa personne et son oeuvre, 2nd ed. (Paris: Cerf, 2002), 634-38.

(18) For more detailed expositions of the Thomistic texts following a chronological order, see Richard Taylor, "Intellect as Intrinsic Formal Cause in the Soul according to Aquinas and Averroes," in The Afterlife of the Platonic Soul: Reflection on Platonic Psychology in the Monotheistic Religions, ed. John Dillon and Maha El-Kaisy Friemuth (Leiden: Brill, 2009), 187-220; and Bazan, "The Creation of the Soul."

(19) For a more detailed accounts of the mechanisms of cognition, see, for instance, Eleonore Stump, Aquinas (London: Routledge, 2003), 244-76.

(20) See, for instance, ST I, q. 78, a. 3: "Est autem sensus quaedam potentia passiva, quae nata est immutari ab exteriori sensibili."

(21) See De anima 2.12.424a17-24.

(22) See Sheldon M. Cohen, "St. Thomas Aquinas on the Immaterial Reception of Sensible Forms," The Philosophical Review 92 (1983): 193-209; Paul Hoffman, "St. Thomas Aquinas on the Halfway State of Sensible Being," The Philosophical Review 99 (1990): 73-92; Robert Pasnau, Theories of Cognition in the Later Middle Ages (New York: Cambridge University Press, 1997), 31-47; Myles Burnyeat, "Aquinas on 'Spiritual Change' in Perception," in Ancient and Medieval Theories of Intentionality, ed. Dominik Perler (Leiden: Brill, 2001), 129-53; Paul Hoffman, "Aquinas on Spiritual Change," Oxford Studies in Medieval Philosophy 2 (2014): 98-103.

(23) St. Thomas Aquinas, Sentencia libri De anima (hereafter, InDA), Opera Omnia iussu Leonis XIII P. M. edita, Tomus 45, 1 (Rome: Commissio Leonina, 1984), bk. 2, c. 24: "Et per hunc modum, sensus recipit formam sine materia, quia alterius modi esse habet forma in sensu, et in re sensibili. Nam in re sensibili habet esse naturale, in sensu autem habet esse intentionale et spirituale."

(24) ST I q. 78, a. 3; InDA, bk. 2, c. 14.

(25) By appealing to its intentional being, Stump claims that sensible species is an immaterial form consisting in the matter of an organ of the body. See Stump, Aquinas, 254. The term "immaterial" is misleading here. Although Aquinas does mention the immaterial existence of sensible species, it does not follow that he views sensation as a wholly immaterial and incorporeal process. See Pasnau, Theories of Cognition, 42-47. Moreover, it will be clear later that for Aquinas what impedes understanding is the materiality of a thing. If the sensible species is already immaterial, there will be nothing preventing it from becoming an intelligible form in actuality, which will destroy Aquinas's sharp distinction between sensual and intellectual cognition.

(26) InDA, bk. 2, c. 5.

(27) ST I, q. 78, a. 4.

(28) InDA, bk. 2, c. 5.

(29) ST I, q. 85, aa. 1-2; InDA, bk. 3, c. 2 (following the chapter numbering of Gauthier's Leonine edition).

(30) St. Thomas Aquinas, Liber de veritate catholicae Fidei contra enrrores infidelium sen Summa contra Gentiles (hereafter, SCG), ed. P. Marc et al. (Turin: Marietti, 1961), bk. 2, c. 59, see also ST I, q. 85, a. 1.

(31) ST I, q. 85, a. 1, ad 3: "colores habent eundem modum existendi prout sunt in materia corporali individuali, sicut et potentia visiva, et ideo possunt imprimere suam similitudinem in visum. Sed phantasmata, cum sint similitudines individuorum, et existant in organis corporeis, non habent eundem modum existendi quem habet intellectus humanus, ut ex dictis patet; et ideo non possunt sua virtute imprimere in intellectum possibilem."

(32) See especially Therese Cory's critique of the standard account of abstraction either as a process of selective attention to some aspects of the phantasm or as a process of stripping the material features of the phantasm, in "Rethinking Abstractionism: Aquinas's Intellectual Light and Some Arabic Sources," Journal of the History of Philosophy 53, no. 4 (2015): 607-46, esp. 620.

(33) ST I, q. 85, a. 2, ad 3.

(34) DUI, c. 3, par. 61: "Manifestum est enim quod hie homo singularis intelligit: numquam enim de intellectu quereremus nisi intelligeremus; nec cum querimus de intellectu, de alio principio querimus quam de eo quo nos intelligimus." See also SCG, bk. 2, c. 59; InDA, bk. 2, c. 27.

(35) See, for instance, ST I, q. 76, a. 1: "experitur enim unusquisque seipsum esse qui intelligit." For more references to this sort of experience and an interesting study of the verb "experiri" in Aquinas's works, see Ruedi Imbach, '"Expertus sum'. Vorlaufige Anmerkungen zur Bedeutung des Verbs 'experiri' bei Albert dem Grossen, Siger von Brabant und Thomas von Aquin," in Les innovations du vocabulaire latin a la fin du moyen age: autourdu Glossaire du latin philosophique, ed. Olga Weijers, Iacopo Costa, and Adriano Oliva (Turnhout: Brepols, 2010), 61-88, esp. 77-86.

(36) See ST I, q. 76, a. 1: "ipse idem homo est qui percipit se et intelligere et sentire." Cory thinks that here perception is used as a general term of cognition, which also indicates the intimate presence of the object to the perceiving person. See Cory, Aquinas on Human Self Knowledge, 71-74. Moreover, as Ruedi Imbach observes, unlike Descartes's fascination with the "ego" in this sort of perception or experience, Aquinas prefers to use the verb "experiri" in its third-person singular form (experitur) or first-person plural form (experimur), which also contributes a significant difference between two approaches to the cognition of the self. See Imbach, '"Expertus sum'," 77.

(37) See also Robert Pasnau, Thomas Aquinas on Human Nature: A Philosophical Study of Summa Theologiae la 75-89 (New York: Cambridge University Press, 2002), 338.

(38) See, for instance, Adams and Trifogli, "Whose Thought Is It?" 625. One noticeable exception is the anonymous manuscript edited by Maurice Giele, Quaestiones in Aristotetis libros I et II De anima, bk. 2, chap. 4, p. 75: "Unde, quod homo proprio sermone intelligit, non concedo; illo tamen concesso, nescio respondere; sed istud nego et merito; ideo faciliter respondebo."

(39) As de Libera rightly notes, Averroes himself introduces the two-subjects theory to explain not how a human being thinks, but rather how different human beings can think about the same thing. However, Aquinas's arguments based upon the dictum hie homo intelligit. were so influential that all Latin Averroists have to "expliquer en quoi l'homme individuel pense, s'il n'est pas le sujet de la pensee." See de Libera, Archeologie du sujet III, 1, 186; 246.

(40) DUI, c. 3, par. 62: "dixit [sc. Auerroys] quod intelligere illius substantie separate est intelligere mei uel illius, in quantum intellectus ille possibilis copulatur michi uel tibi per fantasmata que sunt in me et in te. Quod sic fieri dicebat: species enim intelligibilis que fit unum cum intellectu possibili, cum sit forma et actus eius, habet duo subiecta, unum ipsa fantasmata, aliud intelleetum possibilem. Sic ergo intellectus possibilis continuatur nobiscum per formam suam mediantibus fantasmatibus; et sic dum intellectus possibilis intelligit, hie homo intelligit."

(41) Aquinas, DUI, c. 3, par. 63: "secundum autem dictum Auerroys, intellectus non continuaretur homini secundum suam generationem, sed secundum operationem sensus."

(42) Aquinas, SCG, bk. 2, c. 59, pars. 15-16, see also InSent, bk. 2, d. 17, q. 2, a. 1.

(43) DUI, c. 3, par. 64: "Manifestum est enim quod species intelligibilis secundum quo est in in fantasmatibus, est intellecta in potentia; in intellectu autem possibili est secundum quod est intellecta in actu, abstracta a fantasmatibus." It is interesting to note that Aquinas talks about the intelligible species in phantasms. This is an unusual usage Aquinas concedes for the sake of argument. He immediately revises it in the mirror analogy that follows to emphasize that the intelligible species only exists in the possible intellect.

(44) De Libera argues that Averroes does not conceive of phantasms as a subject-substratum, but rather as a mover that cooperates with the agent intellection in making a human person think. See Averroes, Commentarium magnum 3.4 and de Libera's comments in Archeologie du sujet III, 1, 207-14.

(45) DUI, c. 3, par. 64: "Nisi forte dicatur quod intellectus possibilis continuatur fantasmatibus sicut speculum continuatur homini cuius species resultat in speculo; talis autem continuation manifestum est quod non sufficit ad continuationem actus. Manifestum est enim quod actio speculi, que est representare, non propter hoc potest attribui homini: unde nec actio intellectus possibilis propter precictam copulationem posset attribui huic homini qui est Sortes, ut hie homo intelligit."

(46) See Adams and Trifogli, "Whose Thought Is It?" 628-31.

(47) Deborah Black argues that Averroes never draws any comparison between the eye and the material intellect that would justify Aquinas's presumption here, but rather compares the material intellect with the transparent medium in visual perceptions. Deborah Black, "Models of the Mind: Metaphysical Presuppositions of the Averroist and Thomistic Accounts of Intellection," Documenti E Studi Sulla Tradizione Filosofica Medievale 15 (2004): 319-52. For Aquinas's own understanding of the role of the transparent medium in this analogy, see, for instance, SCG, bk. 2, c. 59. For comments on the analogy of light in understanding the agent intellect's role in abstraction by Averroes, Avicenna, and Aquinas, see Cory, "Rethinking Abstractionism," 614-23.

(48) DUI, c. 3, par. 65.

(49) See, for instance, Aquinas, Quaestiones disputatae de anima (hereafter, QDA), in Opera Omnia iussu Leonis XIII P. M. edita, Tomus 45, 1 (Rome: Commissio Leonina, 1996), a. 19: "Et hoc est quod philosophus dicit, in libro de somno et vigilia (454a8) quod cuius est potentia eius est actio." For comments on the significance of this principle in establishing the agency of human being in his actions, see de Libera, Archeologie du sujet I, 53-59.

(50) DUI, c. 3, par. 66.

(51) Metaphysics 8.6.1045a8-25. See DUI, c. 3, par. 68.

(52) DUI, c. 3, par. 68.

(53) DUI, c. 3, par. 70, see also SCG, bk. 2, c. 73; ST I, q. 85, a. 2.

(54) DUI, c. 3, par. 71.

(55) DUI, c. 3, par. 72.

(56) DUI, c. 3, par. 73.

(57) See, for instance, SCG, bk. 2, c. 76: "Oportet igitur quod principia quibus attribuuntur hae actiones, scilicet intellectus possibilis et agens, sint virtutes quaedam in nobis formaliter existens." For a comprehensive analysis of this principle of intrinsic formal cause in Aquinas's different works, see Taylor, "Intellect as Intrinsic Formal Cause," 190-202. We shall return to this principle below.

(58) Nicomachean Ethics 9.4.1166a15-17. See DUI, c. 3, par. 74.

(59) DUI, c. 3, par. 75: "nulla pars corporis potest diffiniri sine parte aliqua anime." Aquinas's point in this passage is that this intimate relation between the body and the soul shows that a human being cannot be merely his intellect. Nevertheless, it can be read from another direction to show that the soul is ontologically prior to the body.

(60) InDA, bk. 2, c. 2. For a more detailed account of the homonymy of the body and the ontological priority of the soul to the body, see my article "The Ontological Status of the Body in Aquinas's Hylomorphism" (forthcoming in Studia Neoaristotelica 14 [2017]).

(61) DUI, c. 3, par. 78: "sequitur quod intellectus sic uniatur nobis ut uere ex eo et nobis fiat unurn; quod uere non potest esse nisi eo modo quo dictum est, ut sit scilicet potentia anime que unitur nobis ut forma."

(62) DUI, c. 5, par. 95. See also InSent, bk. 2, d. 17, q. 2, a. 1, arg. 1; SCG, bk. 2, c. 75.

(63) DUI, c. 5, par. 96. See also Siger of Brabant, Questiones In III De anima, q. 9, as cited in de Libera, L'Unite de l'Intellect, 400.

(64) ST I, q. 76, a. 1.

(65) For more details about the sources of this principle and its application in various modes of cognition, see John F. Wippel, "Thomas Aquinas and the Axiom 'What Is Received Is Received According to the Mode of the Receiver,'" in Metaphysical Themes in Thomas Aquinas II (Washington, D.C.: The Catholic University of America Press, 2007), 113-22.

(66) This objection is reconstructed from SCG, bk. 2, c. 75; ST I, q. 76, a. 2; QI)SC, a. 9, arg. 13; and DUI, c. 5, par. 102.

(67) DUI, c. 5, par. 100. See also QDSC, a. 9, arg. 3; STI, q. 76, a. 2.

(68) InSent, bk. 2, d. 17, q. 2, a. 1, ad 1: "Ad primum ergo dicendum, quod intellectus non negatur esse forma materialis quin det esse materiae sicut forma substantialis quantum ad esse primum; et ideo oportet quod ad divisionem materiae, quae causat diversa individua, sequatur etiam multiplicatio intellectus, idest animae intellectivae. Sed dicitur immaterialis [provisionary Leonine edition: hoc dicitur] respectu actus secundi, qui est operatio: quia intelligere non expletur mediante organo corporali, et hoc contingit quia ab essentia animae non exit operatio nisi mediante virtute ejus vel potentia; unde cum habeat quasdam vires que non sunt actus quorundam organorum corporis, oportet quod quedam operationes animae sint non mediante corpore." The English translation cited (with slight modifications) is that of Richard Taylor in Ph ilosophical Psychology in Arabic Thought and the Latin Aristotelianism of the 13th Century, 292. The Latin text is the provisionary Leonine edition Taylor uses with one exception noted above.

(69) InSent, bk. 2, d. 17, q. 2, a. 1, arg. 1: "anima rationalis vel intellectus sit unus numero in omnibus."

(70) See, for instance, InSent, bk. 1, d. 3, q. 4, a. 2; ST I, q. 77, a. 1.

(71) In a recent survey on the medieval controversy on the soul's faculties and its essence, Dominik Perler challenges the traditional interpretation of Aquinas's position as maintaining a real distinction between the soul and its faculties. Perler argues that x and y are really distinct only when x can exist without y and y without x. However, the soul can never exist without its faculties as its necessary accidents (propria), and vice versa. See Dominik Perler, "Faculties in Medieval Philosophy," in The Faculties: A History, ed. Dominik Perler (New York: Oxford University Press, 2015), 97-139, esp. 108-09. I think Perler adopts an unnecessarily strong interpretation of the real distinction. In this context, Aquinas claims that the essence of the soul is really distinct from its capacities or faculties merely in that the distinction does not depend upon our conceptions of them.

(72) InSent, bk. 2, d. 18, q. 2, a. 1, ad 6: "anima rationalis nec ex materia composita est, nec est forma materialis, quasi in materia impressa." The translation is mine.

(73) See InSent, bk. 2, d. 17, q. 1, a. 2, ad 6; ST I, q. 77, aa. 1, 6.

(74) ST I, q. 76, a. 1: "Praeterea, eiusdem est potentia et actio, idem enim est quod potest agere, et quod agit. Sed actio intellectuals non est alicuius coiporis, ut ex superioribus patet. Ergo nec potentia intellectiva est alicuius corporis potentia. Sed virtus sive potentia non potest esse abstractior vel simplicior quam essentia a qua virtus vel potentia derivatur. Ergo nec substantia intellectus est corporis forma." See also DUI, c. 3, par. 81.

(75) ST I, q. 76, a. 1, ad 4: "Ad quartum dicendum quod humana anima non est forma in materia corporali immersa, vel ab ea totaliter comprehensa, propter suam perfectionem. Et ideo nihil prohibet aliquam eius virtutem non esse corporis actum; quamvis anima secundum suam essentiam sit corporis forma."

(76) ST I, q. 75, a. 2, ad 1: "hoc aliquid potest accipi dupliciter, uno modo, pro quocumque subsistente, alio modo, pro subsistente completo in natura alicuius speciei. Primo modo, excludit inhaerentiam accidentis et formae materialis, secundo modo, excludit etiam imperfectionem partis.... Sic igitur, cum anima humana sit pars speciei humanae, potest dici hoc aliquid primo modo, quasi subsistens."

(77) See DUI, c. 3, par. 81: "Anima autem humana, quia secundum suum esse est, cui aliqualiter communicat materia non toatliter comprehendens ipsam, eo quod maior est dignitas huius forme quam capacitas materie."

(78) See, for instance, ST I, q. 11, a. 3.

(79) ST I, q. 50, a. 4. See also QDSC, a. 8. See Giogio Pini, "The Individuation of Angels from Bonaventure to Duns Scotus," in A Companion to Angels in Medieval Philosophy, ed. Tobias Hoffmann (Leiden: Brill, 2012), 79-115, esp. 90.

(80) Metaphysics 5.8.1016b31-35, cited in DUI, c. 5, par. 97: "Vnum autem in V Methaphisice dicitur quadrupliciter, scilicet numero, specie, genere, proportione."

(81) DUI, c. 5, par. 97: "Nec est dicendum quod aliqua substantia separata sit unum tantum specie uel genere, quia hoc non est esse simpliciter unum."

(82) Ibid.: "Nec dicitur aliquid unum numero quia sit unum de numero--non enim numerus est causa unius sed e conuerso--, sed quia in numerando non diuiditur; unum enim est id quod non diuiditur."

(83) St. Thomas Aquinas, Quaestiones disputatae de potentia (hereafter, QDP), in Quaestiones disputatae, t. 2, ed. P. M. Pession (Turin-Rome: Marietti, 1965), 1-276, q. 9, a. 5, ad 13: "in rebus creatis principia individuantia duo habent: quorum unum est quod sunt principium subsistendi (natura enim communis de se non subsistit nisi in singularibus); aliud est quod per principia individuantia supposita naturae communis ab invicem distinguuntur." Cited from Enzo Portalupi, "Das Lexikon der Individuality bei Thomas von Aquin," in Individuum und Individualitdt im Mittelalter, ed. Jan A. Aertsen and Andreas Speer (Berlin: De Gruyter, 1996), 57-73, at 67.

(84) DUI, c. 5, par. 99: "Indiuidue ergo sunt substantie separate et singulares." There seems to be no substantial difference between these two terms in Aquinas's ontology, for instance, the Aristotelian claim here sometimes reads as actiones sunt solum singularium, sometimes as actiones sunt individuorum. For more discussions on Aquinas's usage of these terms, see Portalupi, "Das Lexikon der Individualitat."

(85) DUI, c. 5, par. 98: "Nec etiam hoc uerum est, quod substantia separata non sit singularis et indiuiduum aliquid; alioquin non haberet aliquam operationem, cum actus sint solum singularium, ut Philosophus dicit." See de Libera, L'unite de l'intellect, 408.

(86) DUI, c. 5, pars. 98-99: "Non enim materia est principium indiuiduationis in rebus materialibus, nisi in quantum materia non est participabilis a pluribus, cum sit primum subiectum non existens in alio ... Indiuidue ergo sunt substantie separate et singulars; non autem indiuiduantur ex materia, sed ex hoc ipso quod non sunt nate in alio esse, et per consequens nec participari a multis. Ex quo sequitur quod si aliqua forma nata est participari ab aliquo, ita quod sit actus alicuius materie, ilia potest indiuiduari et multiplicari per comparationem ad materiam." The translation is modified from Ralph Mclnerny's in his Aquinas against the Averroists (West Lafayette, Ind.: Purdue University Press, 2002).

(87) Montague Brown, "St. Thomas and the Individuation of Persons," American Catholic Philosophical Quarterly 65, no. 1 (1991): 29-44, at 41; see also Portalupi, "Das Lexikon der Individuality," 67.

(88) DUI, c. 5, par. 99: "lam autem supra ostensum est quod intellectus est uirtus anime que est actus corporis; in multis igitur corporibus sunt multe anime, et in multis animabus sunt multe uirtutes intellectuales que uocantur intellectus: nece propter hoc sequitur quod intellectus sit uirtus materialis, ut supra ostensum est."

(89) St. Thomas Aquinas, De principiis naturae, from Sancti Thomae de Aquino opera omnia, vol. 43, ed. Roberto Busa (Rome: Editori di San Tommaso, 1976), c. 1: "Et secundum hoc differt materia a subiecto: quia subiectum est quod non habet esse ex eo quod advenit, sed per se habet esse completum, sicut homo non habet esse ab albedine. Sed materia habet esse ex eo quod ei advenit, quia de se habet esse incompletum."

(90) For a general study of the intelligible species in medieval philosophy, see Leen Spruit, Species Intelligibilis: From Perception to Knowledge, Volume One: Classical Roots and Medieval Discussions (Leiden: Brill, 1994). For more specific studies on Thomas Aquinas, see Jeffrey E. Brower and Susan Brower-Toland, "Aquinas on Mental Representation: Concepts and Intentionality," The Philosophical Review 117, no. 2 (2008): 193-243; Elena Baltuta, "Aquinas on Intellectual Cognition: The Case of Intelligible Species," Philosoph ia 41, no. 3 (201.3): 589-602.

(91) DUI, c. 5, par. 106: "Est ergo dicendum secundum sententiam Aristotilis quod intellectum quod est unum est ipsa natura uel quiditas rei; de rebus enim est scientia naturalis et alie scientie, non de speciebus intellectis." See also QDSC, a. 9, ad 6.

(92) See Bernardo-Carlos Bazan, "Intellectum Speculativum: Averroes, Thomas Aquinas, and Siger of Brabant on the Intelligible Object," Journal of the History of Philosophy 19, no. 4 (1981): 425-46. See also de Libera, L'unite de I'intellect, 440-42.

(93) For criticisms of the traditional reading in favor of the identity between the intelligible species and the essence of extramental objects, see Claude Panaccio, "Aquinas on Intellectual Representation," in Ancient and Medieval Theories of Intentionality, ed. Dominik Perler (Leiden: Brill, 2001), 185-201; and Robert Pasnau, Theories of Cogni tion, esp. 195-219. For a recent defense of traditional realist reading, see Baltuta, "Aquinas on Intellectual Cognition."

(94) DUI, c. 5, par. 107: "Hec [sc. species] autem, cum sit abstracta a principiis indiuidualibus, non representat rem secundum condiciones indiuiduales, sed secudnum naturam uniuersalem tantum."

(95) See, for instance, Bazan, "Intellectum Speculativum," 436; Baltuta, "Aquinas on Intellectual Cognition," 591.

(96) Aquinas explicitly acknowledges his indebtedness to Avicenna in this regard. See InSent, bk. 2, d. 17, q. 2, a. 1, ad 3. See also Avicenna, Metaphysics, 5.1, as cited in Taylor, "Aquinas and the 'Arabs'," 156.

(97) DUI, c. 5, par. 108: "Est ergo unum quod intelligitur et a me et a te, sed alio intelligitur a me et alio a te, id est alia specie intelligibili; et aliud est intelligere meum et aliud tuum; et alius est intellectus meus et alius tuus."

(98) DUI, c. 5, par. 106: "Hec autem species non se habent ad intellectum possibilem ut intellecta, ... nisi in quantum intellectus reflectitur supra se ipsum." For an interesting account of the significance of the intellect's reflexivity in Aquinas's conception of human agency, see Therese Coiy, "The Reflexivity of Incorporeal Acts as Source of Freedom and Subjectivity in Aquinas," in Subjectivity and Selfhood in Medieval and Early Modern Philosophy, ed. Jari Kaukua and Tomas Ekenberg (Dordrecht: Springer, 2016), 125-41.

(99) DUI, c. 5, par. 108: "Non enim singularitas repugnat intelligibilitati, sed materialitas: unde, cum sint aliqua singularia immaterialia, sicut de substantiis separatis supra dictum est, nichil prohibet huiusmodi singularia intelligi."

(100) DUI, c. 5, par. 100: "Vnumquodque enim sic est ens sicut unum, ut dicitur in IV Methaphisice; sicut igitur esse anime est quidem in coipore in quantum est forma corporis, nec est ante corpus, tamen destructo corpore adhuc remanet in suo esse: ita unaqueque anima remanet in sua unitate, et per consequens multe anime in sua multitudine."

(101) See, for instance, ST I, q. 75, a. 2; QDSC, a. 2.

(102) See n. 84 above.

(103) See, for instance, ST I, q. 39, a. 5, ad 1. For more references, see Bazan, "The Creation of the Soul," 533 n. 51.

(104) ST I, q. 29, a. 2.

(105) ST I, q. 75, a. 2, ad 2: "Potest igitur dici quod anima intelligit, sicut oculus videt, sed magis proprie dicitur quod homo intelligat per animam." The translation cited is from The Treatise on Human Nature: Summa Theologiae la75-89, trans. Robert Pasnau (Indianapolis: Hackett, 2002).

(106) InDA, bk. 3, c. 1: "Intellectus ergo possibilis est, quo hie homo, formaliter loquendo, intelligit." See also QDSC, a. 2; QDA, a. 5; ST I, q. 76, a. 1. See above n. 57.

(107) See, for instance, QDSC, a. 2: "Nulla autem operatio conuenit alicui nisi Peraliquam formam in ipso existentem, uel substantialem uel accidentalem, quia nichil agit aut operatur nisi secundum quod est actu; est autem unumquodque actu per formam aliquam, uel substantialem uel accidentalem, cum forma sit actus."

(108) See ST I, q. 76, a. 1: "Hoc ergo principium quo primo intelligimus, sive dicatur intellectus sive anima intellective, est forma corporis." However, even in this context, Aquinas also carefully mentions the distinction between the intellect as a cognitive power and the intellective soul as the substantial form of human beings. Unfortunately, this subtle distinction has been ignored in Taylor's account mentioned in n. 57.

(109) DUI, c. 4, par. 84: "intellectus formaliter ei (sc. homo singularis intelligens) inhereat: non quidem ita quod sit forma corporis, sed quia est uirtus anime que est forma corporis."

(110) See QDSC, a. 10: "Omne autem agens quamcumque actionem habet formaliter in se ipso uirtutem que est talis actionis principium."

(111) See in particular n. 77.

(112) See de Libera, Archeologie du sujet III, 1, 178.

(113) InSent., bk. 2, d. 17, q. 2, a. 1, ad 1; see n. 68. See also De principiis naturae, c. 1.

(114) For a more nuanced argument for this claim, see Wu, "The Ontological Status of the Body." For a different approach, see John F. Wippel, The Metaphysical Thought of Thomas Aquinas: From. Finite Being to Uncreated Being, (Washington, D.C.: The Catholic University of America Press, 2000), 312-27.

(115) De principiis naturae, c. 1: "Et quia forma facit esse in actu, ideo forma dicitur esse actus."

(116) For a superb account of Aquinas's theory of the unity of substantial form, see John F. Wippel, "Thomas Aquinas and the Unity of Substantial Form," in Philosophy and Theology in the Long Middle Ages: A Tribute to Stephen F. Brown, 117-54.

(117) DUI, c. 2, par. 50: "aliud utique erit ego et michi esse. Et ego quidem est compositus intellectus ex potentia et actu, michi autem esse ex eo quod actu est."

(118) Ibid.: "Esse igitur michi ab anima et hac non omni; non enim a sensitiua, materia enim erat fantasie; neque rursum a fantastica, materia enim erat potentia intellectus; neque eius qui potentia intellectus, materia enim est factiui. A solo igitur factiuo est michi esse."

(119) See, for instance, ST I, q. 75, a. 4.

(120) SCG, bk. 2, c. 58: "Ab eodem aliquid habet esse et unitatem: unum enim consequitur ad ens. Cum igitur a forma unaquaeque res habeat esse, a forma etiam habebit unitatem. Si igitur ponantur in homine plures animae sicut, diversae formae, homo non erit unum ens, sed plura." The English translation is Wippel, The Metaphysical Thought, 338.

(121) Aquinas's obscure conception of the separated soul is still an issue of living debates, especially in regard to whether it is sufficient for the survival of a human person. For a recent account of survivalism, see Eleonore Stump, "Resurrection and the Separated Soul," in Oxford Handbook of Aquinas, ed. Brian Davies and Eleonore Stump (New York: Oxford University Press, 2012), 458-66. For a different corruptionist approach to the problem, see Patrick Toner, "St. Thomas Aquinas on Death and the Separated Soul," Pacific Philosophical Quarterly 91, no. 4 (2010): 587-99; Turner Nevitt, "Survivalism, Corruptionism, and Intermittent Existence in Aquinas," History of Philosophy Quarterly 31, no. 1 (2014): 1-19. This controversy is significant for a diachronic account of personal identity. For our current purposes, it is sufficient to mention that even survivalists concede that the separated soul has a mode of existence totally different from that of a living person.

(122) See, for instance, InSent, bk. 1, d. 8, q. 5, a. 2, ad 1: "Anima est forma absoluta, non dependens a materia, quod convenit sibi propter assimilationem et propinquitatem ad Deum; ipsa habet esse per se, quod non habent aliae formae corporales." InSent, bk. 2, d. 3, q. 1, a. 6: "Anima autem rationalis habet esse absolutum, non dependens a materia." Both texts are cited from de Libera, Archeologie du sujet III, 1, 405.

(123) For instance, de Libera cites Bazan to argue that Aquinas's conception of form does not accord with a strict notion of substantial form. See Archeologie du sujet III, 1, 405-07.

(124) See, for instance, Christopher Shields, "The Priority of Soul in Aristotle's De Anima: Mistaking Categories?" in Body and Soul in Ancient Philosophy, ed. Dorothea Frede and Burkhard Reis (Berlin: De Gruyter, 2009), 156-68.

(125) See, for instance, ST I, q. 75, a. 2: "Nihil autem potest per se operari, nisi quod per se subsistit."

(126) ST I, q. 75, a. 2, ad 2: "Sed per se existens quandoque potest dici aliquid si non sit inhaerens ut accidens vel ut forma materialis, etiam si sit pars."

(127) For a more detailed analysis of Aquinas's doctrine of the creation of the soul, though with a very different evaluation of Aquinas's accounts for the individuality of thinking, see Bazan, "The Creation of the Soul."

(128) This research is a part of the program "Immateriality, Thinking and the Self in the Philosophy of the Long Middle Ages," funded by the British Academy through an International Partnership and Mobility Grant. This research is also funded by the National Social Science Foundation of China ([TEXT NOT REPRODUCIBLE IN ASCII] M Project No. 11CZX042).
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Title Annotation:St. Thomas Aquinas
Author:Wu, Tianyue
Publication:The Review of Metaphysics
Article Type:Essay
Date:Sep 1, 2017
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