Ernst Cassirer was the last of the major exponents of the Marburg School of Neo-Kantianism. This book presents his philosophy of science in the context of the developments in the physical sciences in this century. Cassirer's call for a redefinition of the «concept of substance» is critically evaluated in terms of the meanings of such terms as «laws, » «theories, » and «causality» as used in the sciences. By treating the sciences as one of the Symbolic Forms, this book establishes the relevancy of the critical philosophy of Kant to an understanding of the recent history and philosophy of science.