In “Metaphysics,” the first topic chosen is an old and traditional one—the nature of the self, the status of the individual. Nozick calls up an old and fascinating example: the ship of Theseus. If a plank wears out on the ship, it is thrown away and a new plank is fitted to the deck. Suppose all the planks and all the other equipment of the ship are similarly replaced: Does one still have the old ship, or is it a new one? As if that were not puzzling enough, suppose someone sneaks away with the discarded equipment and forms it into a ship again: Which ship is the old ship and which is the new one?

The problem is not concerned with ships but with people. The components of the body change constantly. Is one still the same person? This problem has to do with the body as material object; if this were the whole problem, it would be easy to solve, simply by denying that molecular interchange constitutes a material change. When mental “objects” are considered, however, the problem is not so easy to dismiss. For example, has a college class on philosophy changed if one of the students is absent? Or (Nozick’s example) if the famous Vienna Circle of philosophers had scattered before World War II (as it did), what would have happened to the Vienna Circle if three members had emigrated to Istanbul and carried on as the Vienna Circle, only to find out at the end of the war that other members had emigrated to the United States and also carried on as the Vienna Circle? Where would the “real” Vienna Circle have been located?

In his solution to this problem, Nozick seems to be analyzing not what the “individual” really is, but what is meant when the word “individual” is used, an approach credited to Ludwig Wittgenstein. Nozick employs the notion of the “closest continuer”; whichever of the entities to be traced bears the closest “continuing” relationship with the original object would be called “the original object.” Thus, in the case of Theseus’ ship, most people would say that the closest continuer would be the ship from which the planks were originally taken and replaced.

The second portion of “Metaphysics,” “Why Is There Something Rather than Nothing,” deals with a famous metaphysical question with which philosophers from Gottfried Wilhelm Leibniz to Martin Heidegger have wrestled. The basic problem in dealing with this question, as Nozick notes, is that the answer to the question explains everything. Yet the question itself is something, part of everything, so how can a question provide an answer for its own existence? Here the problem is identified with the notion of “self-subsumption” and with the further problem, “how is explanation possible?” Here the reader sees Nozick’s motive for dealing with this problem—it provides a philosophical basis for “philosophical explanations.”Is self-subsuming explanation thwarted by the fact that explanations must be deeper than what they (purport to) explain? . . . Explanatory self-subsumption, I admit, appears quite weird—a feat of legerdemain. When we reach the ultimate and most fundamental explanatory laws, however, there are few possibilities.

Either all explanations go round in an infinite circle, each explanation explaining the next, forever, or explanations have a stop; if they stop, where do they stop—at unexplainable “brute facts” or at self-subsuming laws? There are problems with both suggestions, and Nozick wrestles with them. He approaches the matter (with help from his twelve-year-old daughter) and moves toward a statistical notion: There are many ways for something to be something, but only one way for there to be nothing. Thus, if states are assigned randomly, there is a much greater chance for there to be something than there is for nothing. The philosopher can work out a principle that all possibilities will eventually occur: “All possible worlds obtain.” The principle itself is part of a possible world, if it is true, however, so it is self-subsuming, and here one begins to get out of the possibility trap.

In the second large section, “Epistemology,” knowledge and skepticism are covered. How can one convince a skeptic that he is eating an omelet, that he is not, in reality, merely a brain suspended in a vat which is under the impression that “he” is eating an omelet? Again, a classic problem in philosophy. Here Nozick separates the problem into two distinct parts: Can one convince the skeptic that he is wrong? (Answer: no.) Does one have to convince the skeptic that he is wrong if as a philosopher one wishes to deal with the problem? (Answer: again, no.) Here is the basis for Nozick’s preference for explanations over proofs; the skeptic cannot be coerced into belief, but explanation may soften skepticism in others.

In the third large section, “Value,” Nozick wrestles with the foundation of ethics. Ethics has become fashionable again after a long interval of neglect. ( Nozick’s colleague at Harvard, Morton Gabriel White, has also written on ethics.) One of the main concerns of ethics is the determination of value. Value governs choice, and Nozick handles the problem by saying, essentially, that people who behave in an ethically and morally responsible manner are nicer than people who do not. Niceness for Nozick is a powerful criterion, possibly what decency is for George Orwell: a noncoercive standard that may ultimately be the only true guide to action. Saints, martyrs, and heroes end up being used by fanatics to justify their tyranny. Decency and niceness are harder to employ for these purposes. That, presumably, is why Nozick is opposed to any coercive systems, even philosophical ones.