 In the second half of the twentieth century, research in the philosophy of science to a significant extent moved away from foundational concerns. Instead, philosophical questions relating to the growth of scientific knowledge and of scientific understanding became more central. As early as the 1970s, there were voices that argued that a similar shift of attention should take place in the philosophy of mathematics. Lakatos initiated the philosophical investigation of the evolution of mathematical concepts (Lakatos 1976). He argued that the content of a mathematical concept evolves in roughly the following way. A mathematician formulates a deep conjecture, but is unable to prove it. Then counterexamples against the conjecture are found. In response, the definition of one or more central concepts in the conjecture is changed in such a way that the counterexamples are at least eliminated. Still the thus revised conjecture cannot be proved, and gradually new counterexamples appear. The procedure of revising the definition of one or more central concepts is applied again and again, until a proof of the conjecture is found. Lakatos calls this procedure concept stretching. In recent decades, Lakatos’ model of concept change in mathematics has been revised and refined (Mormann 2002).