Recent explorations into Sigmund Freud's intellectual development by Frank Sulloway and Lucille Ritvo have directed attention to the significance of evolutionary theory for psychoanalysis. In this paper I shall pursue the exploration by showing how Darwin was received by members of the so-called Helmholtz circle (Hermann von Helmholtz, Emil du Bois-Reymond, Ernst Brücke) and certain of Freud's teachers in the University of Vienna medical school. I will make the point that the Leibniz–Kant background of these several scientists was important for this reception. I will argue that the Leibniz–Kant tradition came forward to Freud by two roads, Helmholtz's unconscious inference as foundation for a physiology of the senses, and Arthur Schopenhauer's not unrelated uses of the principle of sufficient reason to explain the possibility of lawlikeness in a universe of lawless energies. Finally, I will suggest ways in which Freud received and used the tradition.