Traditionally, a justification for believing something is a priori is if, and only if, it is independent of experience. Throughout Western philosophy since Plato, some of the most divisive questions have been whether a priori justification exists, how it is possible, and how far it reaches. This book is structured around these three main questions.

The first question has recently been modulated as to concern the significance of a priori justification: given the unclarities and presuppositions attached to standard explications of the notion of a priori justification, is it still a useful notion for epistemology? In a tight dialectic, the chapters in this part either attack or defend the theoretical importance of that notion. The second part concerns the sources of a priori justification: since a priori justification is not grounded in experience–our arguably best understood source of justification–in what can it be grounded? The chapters in this part explore the possibility that a priori justification is grounded either in intuition or in understanding. The third part concerns the extent of a priori justification: beyond core cases like mathematical proof, what methods can yield a priori justification? The chapters in this part investigate to what degree and why methods like introspection, testimony, and others have an import on a priori justification. Overall, the book showcases and furthers some of the latest contemporary trends in thinking about these questions.