Many of the pioneers of quantum mechanics — notably Planck, Einstein, Bohr, de Broglie, Heisenberg, Schrödinger, Born, Jordan, Landé, Wigner, and London — were seriously concerned with philosophical questions. In each case one can ask a question of psychological and historical interest: was it a philosophical penchant which drew the investigator towards a kind of physics research which is linked to philosophy, or was it rather that the conceptual difficulties of fundamental physics pulled him willy-nilly into the labyrinth of philosophy? I shall not undertake to discuss this question, but shall cite an opinion of Peter Bergmann, which I find congenial: he learned from Einstein that "the theoretical physicist is … a philosopher in workingman's clothes" ([1],q. v).