Why Do You Do the Things You Do?

THE NATURE OF RATIONALITY By Robert Nozick. 226 pp. Princeton, N.J.: Princeton University Press. $19.95.

ROBERT NOZICK, the Arthur Kingsley Porter Professor of Philosophy at Harvard University, always attacks his problems in a disconcertingly original way. In 1969 he published a discussion of a bizarre imaginary dilemma, thought up by a scientist at the Lawrence Livermore Laboratory in California but unpublished until then, which opened up a new path for research in decision theory (the theory that tries to work out principles for calculating the desirability of alternative courses of action). Five years later, his first book, "Anarchy, State, and Utopia," was widely acclaimed as a tour de force in political thought. Using a range of techniques, from applications of economics to colorful puzzles about men with death rays, it was an extended argument for a near-anarchist "minimal" state. In 1981 came "Philosophical Explanations," a bloated masterpiece groaning with novel suggestions for coping with several of the main topics of academic philosophy. And in 1989, in "The Examined Life," he took on nothing less than the meaning of life, a subject that academic philosophers tended to steer clear of -- and still do, despite his best efforts.

From Mr. Nozick you always expect fireworks, even if some of them go off in their box. His questions, hints, counterarguments and suggestions come so thick and fast that it is next to impossible to appreciate all of them. Start pondering a sentence and you will find yourself led away prematurely by a parenthetical question; think about the question and you will be dragged into a discursive footnote; from the bowels of the footnote, another parenthetical query will leap out at you. If you escape back to the main argument with your concentration intact (unlikely, after a while), the whole wearing business just starts over again. Yet it is worth the effort -- certainly for regular readers of philosophy, and often for others.

"The Nature of Rationality," Mr. Nozick's latest book, and his most narrowly focused one, happily and untypically manages to concentrate on one topic. One topic, but not just one discipline. The theory of rationality touches many areas of probability: economics, decision theory, anthropology, psychology, biology and computer science. Mr. Nozick pokes into many of them. His learning is deep enough to stop such forays from degenerating into the farces of multidisciplinary cross-dressing that they can easily become in lesser hands.

The questions he addresses are fundamental in the true philosophical sense: Why exactly should we want to act and believe rationally? Why should we formulate principles of action and try to stick to them? The questions are not moral but explicatory. He is not out to argue that unprincipled or irrational behavior is immoral; rather, he invites us to consider what we are trying to do, and what the justification for such behavior is. He argues, plausibly, that "principles . . . provide one means to control and reshape our desires." Among his other main philosophical questions are: What is the connection between rational belief and rational action? And what is the relevance of the fact that rationality is something that has evolved?