A Theory of Everything
Can there be a theory of everything? A typical theory is a theory of some things and not others, even when it is very general. The theory of evolution is a theory of living things; it is not a theory of non-living things. Some things evolve and some do not: hence we need a theory of non-evolution as well as a theory of evolution. Even theories of physics are not theories of everything: the two theories of relativity, special and general, are theories of the motion of material things, but they are not theories of the inner workings of the atom. Even if we could reconcile and combine quantum theory and relativity theory into one theory (“the theory of everything”), we would still not have a theory of absolutely everything: we would not have a theory of the mind or even a theory of life, still less a theory of justice or metaphysical necessity.
A theory tells us the nature of a particular class of things, and that involves distinguishing that class from other classes of things—the things to which it does not apply. It is in the nature of a theory that it applies to a limited domain, because a theory tells us what is distinctive of certain entities—living entities, physical entities, psychological entities, ethical entities, mathematical entities. There cannot be a theory of everything because a theory of everything wouldn’t be a theory of anything: it wouldn’t do the job of a theory. It would just be a list or a bland description or a conjunction of more specific theories. It would fail to provide the contrast that is integral to a theory. The form of a theory is: X works like this, unlike Y. Darwin’s theory tells us how living things work, unlike living things. Einstein’s theory tells us how (certain!) material objects work, unlike mental things. We need a different theory for the contrast class.
You might say that some philosophical theories are theories of everything, say idealism or materialism. Such theories say that everything is mental or everything is physical. There are difficulties in interpreting the content of such assertions (what is meant by “mental” and “material”?), and it often turns out that the theory is not really offered as a theory of absolutely everything—including mathematical entities, possibilia, the non-existent. But what is notable is that such (ostensibly) perfectly general theories are not empirical theories—they are philosophical theories. What is impossible is the notion of an empirical theory of everything—a theory like Darwin’s or Einstein’s but applicable to the whole of reality. We certainly have nothing of this kind, and a physicist’s “theory of everything” would not be such a theory; moreover, there seem to be principled reasons why the idea is empty. We have the phrase “a theory of everything”, but it doesn’t denote anything. It is not therefore something to which we should aspire.