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
ha