In the previous chapter, we focused on two important metaphysical questions in the philosophy of mind. One was the question of whether persons or subjects of experience are identical with their physical bodies, or certain parts of those bodies, such as their brains. The other was the question of whether the mental states of persons, such as thoughts and feelings, are identical with certain physical states of their bodies, such as states of neuronal activity in their brains. Many materialists would endorse positive answers to both of these questions, although later in this chapter we shall encounter a species of materialism which denies that mental states, as we ordinarily conceive of them, really exist at all. But before we examine that position, it is worth remarking that, so long as one is a realist about mental states – that is, so long as one considers that states of thinking and feeling really do exist – one can, for many purposes, afford to remain neutral with regard to the question of whether or not mental states are identical with physical states. There are many issues in the philosophy of mind which we can usefully discuss without presuming to be able to resolve that question. And this is just as well, knowing as we now do how thorny a question it is. One of these issues is that of how we can best characterise and classify the various different kinds of mental state which, if we are realists, we believe to exist.