Before the 20th century, the concept of a priori knowledge - knowledge based in reason and reflection rather than experience - found nearly universal acceptance. By the early 20th century, however, philosophers were skeptical of the idea that there was any nontrivial existence of a priori knowledge. Fifty years later, it was fashionable to doubt it existed at all. The articles in this book tackle a priori knowledge from every angle - does it exist? what might it be like? what is its relation to empirical knowledge? - and other topics of perennial interest.