In this collection of essays one of the preeminent philosophers of science writing today offers a reinterpretation of the enduring significance of logical positivism, the revolutionary philosophical movement centered around the Vienna Circle in the 1920s and '30s. Michael Friedman argues that the logical positivists were radicals not by presenting a new version of empiricism (as is often thought to be the case) but rather by offering a new conception of a priori knowledge and its role in empirical knowledge. This collection will be mandatory reading for any philosopher or historian of science interested in the history of logical positivism in particular or the evolution of modern philosophy in general.