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.