Throughout the whole of man's mental development, which word in itself excludes any repetition or stagnation in history, we can trace action and reaction, or rather, according to Hegel's dialectics, “ progress by contrasts.” Certain ideas take root in humanity. They blossom, bear fruits, and then die away. Similar ideas shoot up again; not the same, but full of a new vigour and vitality, rooted in an altogether changed soil; composed of the intellectual blossoms and fruits of a previous era, nourished by the totally different mode of thinking, the increased or decreased amount of knowledge of new generations. This was the case with the acting and counteracting movements of idealism and realism in Greece. The ideas of Demokritos or Hippokrates were superseded by the idealistic arguments of Sokrates, whose principles formed the basis of the philosophies of Plato and Aristotle in the following century. The supernaturalism of Plato and the realism of Aristotle both had their followers, who read and understood them differently. We have first the celebrated physicist Strato, who looked upon the vôs of Aristotle as a mere consciousness of impressions. The activity of the soul was with him simply motion. He explained all existence and life as originating in the natural forces with which matter is endowed. Strato led to Epikurus, who was counteracted by the Stoiks, who, despite their professed idealism, were the most prominent materialists in physical science. We can trace an analogous phenomenon in our own times in which it often occurs, that men with theologically biased minds in metaphysical matters are the most pronounced realists where natural sciences are concerned.