Thales, of Miletus, an Ionian geometrician and astronomer, about whose age considerable uncertainty prevails, but who seems to have flourished towards the close of the seventh century before our era, is by general consent regarded as the father of Greek physical philosophy. Others before him had attempted to account for the world’s origin, but none like him had traced it back to a purely natural beginning. According to Thales all things have come from water. That the earth is entirely enclosed by water above and below as well as all round was perhaps a common notion among the Western Asiatics. It was certainly believed by the Hebrews, as we learn from the accounts of the creation and the flood contained in Genesis. The Milesian thinker showed his originality by generalising still further and declaring that not only did water surround all things, but that all things were derived from it as their first cause and substance, that water was, so to speak, the material absolute. Never have more pregnant words been spoken; they acted like a ferment on the Greek mind; they were the grain whence grew a tree that has overshadowed the whole earth. At one stroke they substituted a comparatively scientific, because a verifiable principle for the confused fancies of mythologising poets. Not that Thales was an atheist, or an agnostic, or anything of that sort. On the contrary, he is reported to have said that all things were full of gods; and the report sounds credible enough. Most probably the saying was a protest against the popular limitation of divine agencies to certain special occasions and favoured localities. A true thinker seeks above all for consistency and continuity. He will more readily accept a perpetual stream of creative energy than a series of arbitrary and isolated interferences with the course of Nature. For the rest, Thales made no attempt to explain how water came to be transformed into other substances, nor is it likely that the necessity of such an explanation had ever occurred to him. We may suspect that he and others after him were not capable of distinguishing very clearly between such notions as space, time, cause, substance, and limit. It is almost as difficult for us to enter into the thoughts of these primitive philosophers as it would have been for them to comprehend processes of reasoning already familiar to Plato and Aristotle. Possibly the forms under which we arrange our conceptions may become equally obsolete at a more advanced stage of intellectual evolution, and our sharp distinctions may prove to be not less artificial than the confused identifications which they have superseded.