 Recent history of philosophy of science has become increasingly aware of neo-Kantianism’s influence on philosophy of science in the twentieth century. It is thus worth taking seriously Cohen’s role in shaping twentieth-century philosophy of science. His commitment to the transcendental method lead him to a view of epistemology, and theoretical philosophy more generally, as essentially a philosophically critical account of the historical development of concepts in the mathematically-precise natural sciences. This view of a historically-oriented philosophy of science is nowhere more clear in Cohen’s work than in his Principle of the Infinitesimal Method. There are two significant points about the view he expresses there. First, for Cohen, the theory of knowledge begins by accepting a body of existing science, and then gives a philosophical reconstruction of important concepts and developments in that science’s history. Second, Cohen thinks the principal topic of the theory of knowledge is mathematically-precise natural science—paradigmatically mathematics and physics. Cohen’s disciple, Ernst Cassirer, would carry out this program in, among other writings, three books on the history and philosophy of physics.