Isaiah Berlin.

ISAIAH Berlin is without a doubt the greatest living authority on the history of ideas. His historical inquiries into the works of a remarkably diverse group of thinkers and philosophical concepts constitute one of the intellectual treasures of our time.

Yet since his early days as an abstract philosopher at Oxford, Berlin has made almost no forays into pure political philosophy. The closest he has come to a comprehensive statement of his political philosophy is his famous essay "Two Concepts of Liberty," which gave currency to the distinction between negative and positive liberty and argued that negative liberty is the more worthy of defense. Even so, he stands as one of the pre-eminent modern exemplars of political liberalism -- though Berlin's liberalism, rooted firmly in the values of the Enlightenment, is of such catholicity and so devoid of partisanship that it bears little resemblance to most of what today passes for liberalism. It is the singular merit of John Gray to have distilled from Berlin's voluminous writings the philosophical essence of his liberalism within this slim volume.

Gray, whose earlier works include a study of the philosophy of another eminent liberal, F. A. Hayek, is well qualified to write about Berlin, having disavowed his own liberalism largely owing to the destructive criticisms made by the eighteenth-century Counter-Enlightenment Romantics. While he is primarily concerned with explicating Berlin's views and is admirably reticent about interjecting his own, he is keenly aware of how the clash between the Enlightenment and the Counter-Enlightenment that Berlin portrays in his historical studies has led Berlin to alter his justification for liberal values from the one offered by the Enlightenment.

In their hopeful view of human nature and the possibility for human improvement, the Enlightenment philosophers believed that, guided by reason alone, men could set themselves free from the ancient bonds of ignorance, superstition, and despotism. Unleashed from the shackles of oppression, people everywhere could join together in building a universal society based on liberty, justice, and tolerance.

The Romantic Counter-Enlightenment figures corrosively attacked this view. Individuals, argued the Romantics, are neither inherently rational nor fundamentally good. Nor are they enslaved by despotic governments and misled by organized religion, as Enlightenment thinkers alleged. Rather, they are weak, impulsive, and confused, formed by their environment, molded by their history, controlled by their instincts, and driven by their passions. Reason alone cannot teach us to distinguish between good and bad or to tell right from wrong, much less cause us to act accordingly.

Accepting much of this critique, Berlin acknowledges that we cannot rationally determine what is good and right in all cases. The tragic and unalterable feature of our moral landscape is that we must often choose between conflicting and irreconcilable moral goods and moral values. These irreconcilable conflicts demand radical, even tragic, choices that are expressions of our existential will, not the dictates of our reason.

However, in Berlin's view, it is by making such morally indeterminate choices that we are able to transform ourselves. For Berlin, the basis within the liberal tradition for the preferred status of negative liberty -- the absence of human constraint on our choices -- is precisely liberty's enlargement of the opportunity for self-transformation.

Unlike the Enlightenment philosophers, who believed in a universal scale of values by which all moral questions could be rationally decided, Berlin acknowledges that the choice of liberty and other liberal values entails the sacrifice of other worthwhile values or ways of life that are associated with goods that are irreconcilable with liberal values. If we nevertheless choose those liberal values, it must be because liberalism is itself a tradition -- our tradition -- and a way of life -- our way of life -- that we, perhaps irrationally, prize, a tradition with roots much older than the Enlightenment, extending deep into antiquity. The Enlightenment may have articulated and rationalized this tradition, changing it, possibly distorting it, in the process, but did not create it, much less deduce it from rational principles.

To recognize that liberalism in its broadest sense -- individual liberty, tolerance, equality before the law, democracy -- is not the necessary implication of self-evident, universally valid axioms of human nature, but the contingent outgrowth of millennia of historical experience, and still to embrace it and defend it as an ideal and as a way of life, is what, in the words of Joseph Schumpeter quoted by Berlin in his essay on liberty, distinguishes a civilized man from a barbarian. In his thoughtful study of Berlin's political philosophy, John Gray performs the admirable service of demonstrating how consummately civilized a man Isaiah Berlin is.