Shadows of the Mind: A Search for the Missing Science of Consciousness.

Years ago, one of my professors distributed to his class a collection of articles by philosophers and theologians, on the one hand, and hard scientists, on the other. He asked us to count qualifiers--words like "maybe," "perhaps," "possibly." The results took us by surprise. The philosophers and theologians tended to make unqualified assertions. The hard scientists hedged their statements with probability.

In his new book on human consciousness, Roger Penrose, a physicist and mathematician, is appropriately tentative. His subject is a big one and his speculations bold, but he hedges them with a befitting humility.

Penrose is the Rouse Ball Professor of Mathematics at the University of Oxford. He has worked with Steven Hawking on the structure and history of the universe, he is a computer whiz and the closest thing to an all-round genius we are likely to find. This is his second book to address the problem of explaining consciousness.

The first, The Emperor's New Mind, a rather technical treatise on physics, computers, and the mind, climbed onto the New York Times's best-seller list. Its success undoubtedly reflected Penrose's stature as a thinker, and our own interest in the phenomenon of self-awareness.

In that earlier book, Penrose said he had been advised that each formula or equaltion would cut his readership by half. He ignored that advice, and asked his readers to hang in even in the face of an occasional formula. Apparently enough did to make the book a success.

In the present work, Penrose throws commercial caution to the wind. Shadows of the Mind is profoundly mathematical both in content and spirit. It is not an easy read; indeed, few readers will be qualified by training or temperament to follow the author through all the steps of his argument. However, this remains an important book, with much that is valuable to say about the science of mind.

At issue is whether computers can become conscious or self-aware. Penrose lists four attitudes toward the question:

A. Consciousness is mere computation; the brain is a computer made of meat.

B. Computers may simulate human consciousness without actually being conscious, in much the same way that a computer can simulate a hurricane without actually being a hurricane.

C. Physical activity in the brain is the basis for awareness, but this activity cannot even be simulated computationally.

D. Consciousness cannot be explained by physical, computational, or any other scientific terms.

Most artificial intelligence researchers and neuroscientists hold to A or B, for the obvious reason that these positions offer hope of timely success. People with traditionally religious world views tend to affirm D. Penrose stakes his claim on C. He believes awareness is a manifestation of the physical brain, but that it arises from activity that cannot be programmed into a computer, no matter how complex that machine might be. Attributes of mind such as insight, intuition, and self-awareness will forever transcend any algorithm or computation.

Penrose centers his argument on a famous theorem proved by the Czech-born logician Kurt Godel in 1931. It states that any rigidly logical mathematical system contains statements that cannot be proved or disproved on the basis of axioms within the system. In other words, there are mathematical statements whose truth is accessible to human intuition and insight which cannot be proven by the application of any set of rules. From this Penrose concludes that human thinking contains a noncomputational ingredient.

What might it be? Mind-body dualism? A ghost in the machine? Almost certainly not, says Penrose. Then what? He looks to the strange behavior of the quantum world for his answer.

Computers are binary devices, both in logic and operation. A transistor is either on or off; a data bit is either a one or zero. Elementary particles--photons or electrons for example--follow a different logic. According to the rules of quantum physics, there is a sense in which a particle can be in two places at the same time. The properties or states of quantum systems are said to be entangled, so that even after separation two photons emitted in opposite directions from the same source seem to know what the other is doing, instantly, across centimeters, perhaps even light-years, of space. These mysteries--called Z-mysteries by Penrose, from puZZle--violate classical common sense, but they have been experimentally demonstrated.

Penrose believes the noncomputational qualities of human consciousness spring from Z-mysteries of the quantum world, perhaps having their origin in the microtubules of the cytoskeleton of the brain's neurons. According to Penrose, the leap of imagination, the spark of insight, are macromanifestations of the nonlocal, noncausal, yes-and-no laws of micronature.

The problem is that neither Penrose nor anyone else fully understands how microquantum magic is translated into macrobehavior. What is missing is a satisfactory theory connecting the quantum and classical worlds. We need to understand how the indeterminate quantum behavior of individual elementary particles achieves coherence across broad regions of the brain. Until such a theory is found, Penrose believes, no scientific explanation of human consciousness will be forthcoming.

This is a sketchy summary of a dense and difficult book. Shadows of the Mind is not so much an explanation of consciousness as a program for future research. However, all who are seriously interested in the scientific explanation of consciousness would do well to make themselves aware of Penrose's C-view, as well as A/B works such as Francis Crick's Astonishing Hypothesis and Daniel Dennet's Consciousness Explained. The truth is, we do not yet know enough about consciousness to choose among A, B, and C.

As for D, no scientist I know of seriously entertains the possibility that the mind exists independently of the brain, or that it will forever elude scientific explanation, but even about this possibility Penrose expresses appropriate caution. His broad expertise, incisive intelligence, and philosophical tentativeness are the strengths of this work.