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.