My aim in this book is to explain and defend a rationalist conception of a priori justification and knowledge: a view according to which there is genuine a priori justification that is not limited in its scope to tautologies or matters of definition. Though taken largely for granted throughout most of the history of philosophy, such a view has fallen into increasing disrepute in the last two centuries and has been generally repudiated in recent times. Nonetheless, as explained further in Chapter 1, it is arguably difficult or impossible to make good sense of most if not all claims of empirical knowledge, and indeed of reasoning generally, while eschewing any a priori appeal. What this indicates, I think, is that the prevailing forms of empiricism are in fact untenable, and that a re-examination of rationalism is sorely needed.

Though this book is not primarily meta-philosophical in character, the need for an account of genuine and non-tautological a priori justification seems to me especially urgent for philosophy itself. While it is not my purpose to argue the matter in detail here, my conviction is that philosophy is a priori if it is anything (or at least if it is anything intellectually respectable); and that the practice of even those who most explicitly reject the idea of substantive a priori justification inevitably involves tacit appeal to insights and modes of reasoning that can only be understood as a priori in character, if they are justified at all.