The primary topic of this book is a decade in the philosophical life of what might loosely be called Vienna. Between 1925 and 1935 in the neighborhood of Vienna, the usually sluggish step of the Spirit suddenly quickened as some of his most enlightened voices started talking to one another. Wittgenstein, Tarski, Carnap, Schlick, Popper, and Reichenbach were, perhaps, no wiser than some of their contemporaries, but circumstances led them to interact during that decade, and the result of that dialogue deserves our attention.

When I started writing this book, I intended to explain in the preface that this was the history of epistemology since Kant, the way Carnap would have written it had he been Hegel. Since then I have come to think that while the Spirit may not be malicious, he is certainly forgetful. In Vienna he took a decisive step forward on the subject of the a priori; but he also moved sideways and backward on other crucial matters. Most of his erratic behavior could have been avoided had he been aware of some of his achievements in the preceding century. He may, perhaps, be excused since his best insights were due to the least noticeable of his voices.

Within the field of epistemology one may discern three major currents of thought in the nineteenth century: positivism, Kantianism, and what I propose to call the semantic tradition. What distinguished their proponents primarily was their attitude toward the a priori.