Peirce's metaphysics is among the most difficult and controversial elements of his philosophy. His account of the nature of reality contains ideas that even he would acknowledge test the very limits of understanding. His hope in drafting an account of reality is that other inquirers will take up his ideas, clarify them and develop them further than even he was ever able to. In discussing Peirce's metaphysics I do not assess its contribution to philosophy or even consider its strengths and weaknesses. My goal is rather to show that, for all its obscurity and difficulties, it forms an integral part of his philosophical system and, moreover, an important part of his case against nominalism.

In Chapter 2 I noted that Peirce thinks the science of inquiry affords a secure basis for metaphysical theorizing. In taking this view he stands against nominalists (and others) who dismiss metaphysical questions as unanswerable or senseless. As he sees it, his theory of inquiry – including the pragmatic maxim, the theory of symbols, the principles of abduction, deduction and induction and the theory of truth – holds in any world in which there is a truth to discover. It holds, that is, regardless of what the psychological or biological character of inquirers turns out to be and whatever facts or laws of nature happen to obtain in the actual world.