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.
The teaching of Anaximenes seems at first sight to be [{29}] a falling off from the position of Anaximander, because he goes back to the position of Thales in favour of a determinate matter as first principle. But in one respect at least there is here an advance upon Anaximander. The latter had been vague as to how formless matter differentiates itself into the world of objects. Anaximenes names the definite processes of rarefaction and condensation. If you believe, as these early physicists did, that every different kind of matter is ultimately one kind of matter, the problem of the differentiation of the qualities of the existent elements arises. For example, if this paper is really composed of air, how do we account for its colour, its hardness, texture, etc. Either these qualities must be originally in the primal air, or not. If the qualities existed in it then it was not really one homogeneous matter like air, but must have been simply a mixture of different kinds of matter. If not, how do these properties arise? How can this air which has not in it the qualities of things we see, develop them? The simplest way of getting out of the difficulty is to found quality upon quantity, and to explain the former by the amount or quantity, more or less, of matter existent in the same volume. This is precisely what is meant by rarefaction and condensation. Condensation would result in compressing more matter into the same volume. Rarefaction would give rise to the opposite process. Great compression of air, a great amount of it in a small space, might account for the qualities, say, of earth and stones, for example, their heaviness, hardness, colour, etc.
Hence Anaximenes was to some extent a more logical and definite thinker than Anaximander, but cannot [{30}] compare with him in audacity and originality of thought.
Other Ionic Thinkers
We have now considered the three chief thinkers of the Ionic School. Others there were, but they added nothing new to the teaching of these three. They followed either Thales or Anaximenes in stating the first principle of the world either as water or as air. Hippo, for example, followed Thales, and for him the world is composed of water, Idaeus agreed with Anaximenes that it is derived from air. Diogenes of Apollonia is chiefly remarkable for the fact that he lived at a very much later date. He was a contemporary of Anaxagoras, and opposed to the more developed teachings of that philosopher the crude materialism of the Ionic School. Air was by him considered to be the ground of all things.