Shortly before he died Alberto Coffa said he was writing “a very grumpy book.” It has turned out to be grumpy indeed, but also funny and deep and wonderful. When he began his work nearly 20 years ago he chose what must have seemed the most unpromising topic imaginable: the history of the a priori, both in the nineteenth century and especially among the logical positivists in the decade following 1925. In the early 1970s it was risky for a promising young philosopher of science to concentrate on historical topics. The nineteenth century was, with a few notable exceptions, terra incognita to analytically inclined philosophers. The positivists were pariahs. And many philosophers were convinced that there was no such thing as the a priori to be studied. Times have changed a bit, but this book still covers new territory. And though I cannot accept all of his conclusions, Coffa has much to teach, perhaps especially to those who would have found the whole investigation unpromising.