Aristotle tells us that philosophy begins in wonder. Thales, often said to be the first philosopher, wondered how the world is made, and of what. His answer, "Water," strikes us as charmingly naive. We know so much better. Yet for his day it was a shrewd guess. Water is common, essential to life, and versatile, being the only substance then known to exist in three key forms, as a liquid, a gas and a solid. Positing water as the underlying stuff of which everything is composed therefore had considerable explanatory power and considerable power to unify the phenomena. How the phenomena are, it seemed, is determined by something to do with how water is.

What intrigued Thales' contemporaries and influenced his successors was not his belief that water in particular is the unifying principle. They were excited by the idea that there is an underlying unity, accessible to rational understanding, of which everything else is somehow a manifestation, and a manifestation in such a way that how the underlying stuff is determines how everything else is. Though later philosophers denied that the one stuff is water, they retained the presupposition that despite all differences, everything is a manifestation, not necessarily via composition, of some one kind of existent or some limited variety of kinds, not necessarily kinds of "underlying stuff" or "substance." Many a metaphysician has sought unity in some such limited variety of existents or beings or Being or processes, a unity according to which the properties of these unifiers determine the properties of all the diverse phenomena in which they are manifested. Over the centuries, there have been many candidates for the unifiers. Among philosophers who flourished in just the first century or two after Thales, one finds Anaximander's indefinite, Heraclitus' fire, Parmenides' unchangeable plenum, Democritus' atoms and the void, Pythagoras' numbers, Plato's Forms, and Aristotle's matter and substance plus its movers. Among later philosophers, one encounters Descartes' (1596-1650) two substances (mental and physical), Spinoza's (1632-1677) one substance (God-or-Nature), Hegel's (1770-1831) Spirit, Whitehead's actual occasions of becoming, and so on and on.

Each of these philosophical answers to Thales' question amounts to a theory of being qua being. Each implies that to be is to be made or composed of, or identical with, or in some other fashion a manifestation of the unifiers, in such a way that how the unifiers are determines how everything else is. Some imply further that to be is to be only or nothing but some such affair. Even those that do not imply this mostly assume that the unifiers enjoy an unconditional priority over the mere phenomena that are their manifestations. Discourse about the unifiers is supposed to be privileged over all other discourse and to express the way things really are, the dominant pattern of being, the ultimate character of the world. The vocabulary of the privileged discourse is a "final vocabulary."

The fact that so many varieties of metaphysics have been totalizing or monopolistic in these ways can make it look as though all varieties must be. But we saw in §§1.2 and 2.2 that not every theory of being qua being need be totalizing or committed to some "final vocabulary." Nor need there be such a thing as the way things are, despite the contrary tendency in traditional metaphysics. Here we need to deepen the argument of those earlier sections, by looking at an especially relevant contemporary answer to Thales' question. This is the physicalist answer: to be is to be composed of the entities physics studies, and how all the phenomena are is determined by the properties of the basic physical entities. Physicalism is widely supposed to be the paradigm of a totalizing or monopolistic metaphysics. In this chapter and the next, we find reason to wonder whether it really is, in contrast with neighboring scarecrows. The lessons we learn in this way about physicalism will turn out to apply to nonphysicalist varieties of metaphysics as well. Whatever the fate of physicalism, therefore, we have ample reason to study it for what it can tell us about metaphysics in general.

A physicalist answer to Thales' question, some philosophers think, is supported by current scientific theory. They are encouraged in this belief by several considerations, including the judgments of some physicists themselves. One physicist even goes so far as to claim that "science has solved the Thales' problem ... at least in the sense it was originally posed." In the sense it was originally posed, we are told, the problem was about "the structure and composition of bulk matter, by which is meant the objects and substances we find around us," the everyday furniture of the world. The Thales' project, which "has remained close to the center of interest of physicists for almost 2500 years," is to find an adequate and observationally testable answer to the problem. The answer, we now know, is that the objects and substances we find around us are composed of particles and processes that the laws of contemporary physics are about (basically the laws of quantum mechanics and of the theory of relativity). Light, heat, color, hardness, the shining of the sun, the behavior of liquids, gases and solids, all of chemistry, even life -- all these and more can be accounted for in this way. Underlying the complexities of the everyday or manifest phenomena is a unifying simplicity described by physics.

This way of construing Thales' question restricts its scope to the everyday phenomena. Metaphysicians are among those who widen the scope. The question is to be construed as about not only what we find around us, but everything whatever, including the basic entities and processes posited by physics itself. What are they composed or grouped of? Where do they come from? What explains why they follow the laws they do? Of course these further questions might be based on a false presupposition, a possibility we noted in the last chapter. But even if they are, they make us wonder: in order to see the Thales' project as having been completed in contemporary physics, mustn't we significantly narrow the project's scope? When we ask not only about bulk matter and everyday processes but about everything whatever, isn't physics bound to fall short?

There are philosophers who disagree. They believe that whatever requires accounting for is adequately accounted for in terms of the basic entities and processes physics investigates. Even when the Thales project is accorded the widest scope, a physicalist answer suffices. Such an answer amounts to a theory of being qua being. To be is to be composed of basic physical entities and processes, and in such a way that all the aspects or properties of things are determined by the physical properties of the basic entities and processes. Physicalism is a descendant of the materialism of Democritus, who held that to be is to be a collection of atoms in the void.

Appalling though physicalism is to many people, we need to understand why in our day it is so attractive to others. One reason is that, right or wrong, physicalism is clearly an answer to Thales' question, whether construed narrowly or broadly; some varieties of metaphysics are not. Another reason is that physicalism seems to be supported by the growing success of the sciences in closing explanatory gaps in our understanding of the world. A third is that, unlike some varieties of metaphysics, physicalism does at least address the problem of how to give a plausible account of the relation between scientific and other sorts of truth, a challenge for all varieties of metaphysics we noted in §1.3. A fourth is that, unlike most other theories of being qua being, physicalism is meant to be criticizable by way of observational testing. According to contemporary physicalists, the principles of physicalism are to be treated as high-level empirical hypotheses or generalizations. In this respect they resemble the very abstract and comprehensive principles of theoretical physics. If phenomena turn up that resist a physicalist account even after years of trying, the physicalist's own principles should be rejected or revised. Physicalists are therefore prepared to concede, if need be, that there are more things in heaven and earth than are dreamt of in their philosophy. Or so we are told.