Fragments of Anaximenes
From John Burnet, Early Greek Philosophy, third ed., 1920, chapter 1 section c; full rpt. online at Early Greek Philosophy, Chapter 1C.
Anaximenes is one of the philosophers on whom Theophrastus wrote a special monograph; and this gives us an additional guarantee for the trustworthiness of the tradition. The following are the passages [from ancient authors] which contain the fullest account of the central feature of his system:
Anaximenes of Miletus, son of Eurystratus, who had been an associate of Anaximander, said, like him, that the underlying substance was one and infinite. He did not, however, say it was indeterminate, like Anaximander, but determinate; for he said it was Air. -- Phys. Op. fr. 2 (R. P. 26).
From it, he said, the things that are, and have been, and shall be, the gods and things divine, took their rise, while other things come from its offspring. -- Hipp. Ref. i. 7 (R. P. 28).
"Just as," he said, "our soul, being air, holds us together, so do breath and air encompass the whole world." -- Aet. i. 3, 4 (R. P. 24).
And the form of the air is as follows. Where it is most even, it is invisible to our sight; but cold and heat, moisture and motion, make it visible. It is always in motion; for, if it were not, it would not change so much as it does. -- Hipp. Ref. i. 7 (R. P. 28).
It differs in different substances in virtue of its rarefaction and condensation. -- Phys. Op. fr. 2 (R. P. 26).
When it is dilated so as to be rarer, it becomes fire; while winds, on the other hand, are condensed Air. Cloud is formed from Air by felting; and this, still further condensed, becomes water. Water, condensed still more, turns to earth; and when condensed as much as it can be, to stones. -- Hipp. Ref. i. 7 (R. P. 28).
. . . Rarefaction and Condensation
At first, this looks like a falling off from the more refined doctrine of Anaximander to a cruder view; but this is not really the case. On the contrary, the introduction of rarefaction and condensation into the theory is a notable advance. In fact, it makes the Milesian cosmology consistent for the first time; since a theory which explains everything as a form of a single substance is clearly bound to regard all differences as quantitative. The only way to save the unity of the primary substance is to say that all diversities are due to the presence of more or less of it in a given space. And when once this step has been taken, it is no longer necessary to make the primary substance something "distinct from the elements," to use Aristotle's inaccurate but convenient phrase; it may just as well be one of them.