The teaching of Anaximander exhibits a marked advance beyond the position of Thales. Thales had taught that the first principle of things is water. The formless matter of Anaximander is, philosophically, an advance on this, showing the operation of thought and abstraction. Secondly, Anaximander had definitely attempted to apply this idea, and to derive from it the existent world. Thales had left the question how the primal water developed into a world, entirely unanswered. Anaximenes Like the two previous thinkers Anaximenes was an inhabitant of Miletus. He was born about 588 B.C. and {28} died about 524. He wrote a treatise of which a small fragment still remains. He agreed with Thales and Anaximander that the first principle of the universe is material. With Thales too, he looked upon it as a particular kind of matter, not indeterminate matter as taught by Anaximander. Thales had declared it to be water. Anaximenes named air as first principle. This air, like the matter of Anaximander, stretches illimitably through space. Air is constantly in motion and has the power of motion inherent in it and this motion brought about the development of the universe from air. As operating process of this development Anaximenes named the two opposite processes of (1) Rarefaction, (2) Condensation. Rarefaction is the same thing as heat or growing hot, and condensation is identified with growing cold. The air by rarefaction becomes fire, and fire borne aloft upon the air becomes the stars. By the opposite process of condensation, air first becomes clouds and, by further degrees of condensation, becomes successively water, earth, and rocks. The world resolves again in the course of time into the primal air. Anaximenes, like Anaximander, held the theory of "innumerable worlds," and these worlds are, according to the traditional view, successive. But here again Professor Burnet considers that the innumerable worlds may have been co-existent as well as successive. Anaximenes considered the earth to be a flat disc floating upon air. The origin of the air theory of Anaximenes seems to have been suggested to him by the fact that air in the form of breath is the principle of life.