In the 1950s, George Canguilhem became known in France as a vocal exponent of the philosophy of the concept, an approach to epistemology that treated science as the highest expression of human rationality and scientific concepts as the necessary preconditions for the manifestation of scientific truth. Philosophers of the concept, Canguilhem included, viewed concepts as the key to the study of science; and science, in turn, as the key to a substantive theory of reason. This article explains what concepts are (...) for Canguilhem, how they are extracted from the history of the sciences, and why they continue to matter for contemporary debates in the History and Philosophy of Science. (shrink)