I vividly remember a remark made by Professor Freud on the occasion of my first personal interview with him, which followed one of his important lectures in Worcester, in 1909. We had been talking, obviously—though the details of the conversation I do not distinctly recall—about the significance of his well-known belief that the principle of causality applied as definitely, and, virtually, in the same sense, in the realm of human thought2 as it does in the