  The question permits answers at varying levels of generality. At its most general, it is a request for a theory of types, the way set theory answers the question “what is a set?” A general answer would tell us what sort of thing a type—any type—is. For example, is it sui generis, or a universal, or perhaps the set of its tokens, or a function from worlds to sets, or a kind, or, as Peirce maintained, a law? These options are discussed in §4.1. At a more specific level, “what is a type?” is a request for a theory that would shed some light on the identity conditions for specific types of types, not necessarily all of them. It would yield an account of what a word (or a symphony, a species, a disease, etc.) is. This is in many ways a more difficult thing to do. To see just how difficult it is to give the identity conditions for an individual type, §4.2 considers what a word is, both because words are our paradigm of types, since the type-token distinction is generally illustrated by means of words, and because doing so will show that some of the most common assumptions made about types and their tokens are not correct. It will also illuminate some of the things we want from a theory of types.