We mentioned Heisenberg's speech as a symptom of a certain tendency, characteristic of the latest phase in science, to survey critically its own epistemological foundations. A few years previous to Heisenberg's speech, the need of such a survey found an eloquent advocate in the late Professor A. N. Whitehead, in his book Science and the Modern World, where, in view of the contradictory nature of modern physical theories, he insists that 'if science is not to degenerate into a medley of ad hoc hypotheses, it must become philosophical and enter upon a thorough criticism of its own foundations'.