Heisenberg's discovery of quantum mechanics was accompanied, as we have seen, by a great insight which was of the nature of a profound metaphysical conversion; it was the discovery of a new meaning for reality which was different from that of Newton, Gauss, Maxwell, Hertz and the grand tradition of classical physics. The sudden swing to the crude empiricism which characterised the early days of complementarity did not last in Heisenberg's case, but changed slowly under the subtle polarising influence of a strong rationalist tendency which was in keeping with his temperament and choice and was encouraged by his mathematical powers. Just as Einstein began his philosophic career under the influence of Hume and Mach to end fifty years later in spiritual companionship with Leibniz and Spinoza, so Heisenberg began as a disciple of Hume and ended in the company of Kant. The transition was not sudden like his first conversion, nor was he conscious of the growing complexity of his thought. If he had been, he would have taken greater pains in his later writings to distinguish his early views from his later. The only indication of the metamorphosis that occurred between 1925 and 1955 is in the change in usage of such words as "objectivity", "causality", "reality", etc., which a careful examination of the context reveals. We have described this metamorphosis elsewhere as the conquering of Hume by Kant; we might also summarise his philosophical development as the result of a dialectic between the Plato of his temperament and choice, and the empiricism forced upon him by the discovery of quantum mechanics and by the environment of Copenhagen.