This book provides a detailed and comprehensive account of the problem of a priori knowledge from a historical as well as a systematic perspective. The author explores Kant's views in connection with the possibility of revision, something hardly, if at all, done in philosophical literature. Furthermore, the views of well-renowned philosophers such as Quine, Putnam, Kitcher, and Hale are discussed in detail and are put into a historical and systematic perspective. Finally, this book contains a glossary of important notions offering illuminating accounts of a priori knowledge and related notions and explains the relationship between a priori knowledge, fallibility and revision. The detailing of concepts such as 'defeasibility', 'infallibility', 'falsifiability' helps anyone reading philosophical literature to pin down the meaning of the terms and its implications in this context. The enriched and dual approach the author takes makes the book a very useful and lucid guide to the problem of a priori knowledge.