=Plato’s Conception of Physical Nature.= Plato constructed a rough sketch of the philosophy of nature in his later years, in compliance with the needs of his School, and perhaps with the urging of his pupil, Aristotle. In his earlier period, he would have nothing of physics, and was in this respect quite in accord with the spirit of Socrates. To the end of his life he maintained that there can be no true knowledge of the physical world; for it is a world of change, and therefore all scientific conclusions about it could be only probable. In a mythical account in the _Timæus_ he drew a picture of the constitution of the world. He conceived a Demiurge or world-forming God to exist, and he thought that this God made the world out of not-Being or empty space “with regard to the Ideas.” The world thus constructed is conceived by Plato as a huge living thing, composed of a visible body and an invisible soul. The world-soul sets the world-body in a circular motion, which motion was considered by antiquity to be the most perfect of all motions. In sharp opposition to the mechanical theory of the world, Plato conceived the world to be endowed with knowledge, of which the spherical motion in its return upon itself is the symbol. The world is unitary and unique, the most perfect and most beautiful world, and its origin can be traced only to a reason working toward ends. Plato’s physics, of which the above is an abbreviated account, will be seen to be of little importance; but it was unfortunately, as we have said, this side of his doctrine that was emphasized in the Middle Ages.