This pioneering book demonstrates the crucial importance of Wittgenstein's philosophy of mathematics to his philosophy as a whole. Marion traces the development of Wittgenstein's thinking in the context of the mathematical and philosophical work of the times, to make coherent sense of ideas that have too often been misunderstood because they have been presented in a disjointed and incomplete way. In particular, he illuminates the work of the neglected 'transitional period' between the Tractatus and the Investigations . `Marion's book is an important contribution to the small but growing body of literature on Wittgenstein's philosophy of mathematics. Its value lies especially in the advance it makes over previous work in situating Wittgenstein's philosophy of mathematics not only in its philosophical, but also its mathematical context ... a welcome addition to the literature.' Mark A.Joseph`fine historical study ... Marion tells an interesting and convinving story. He carefully identifies both the continuities and the discontinuities in the evolution of Wittgenstein's ideas, and makes good sense of each ... Marion's book is certainly to be recommended both for its exegesis and, through its compelling presentation of these ideas, for its contribution to the philosophy of mathematics.' A W Moore, Studia Logica Mathieu Marion is Assistant Professor of Philosophy, University of Ottawa.