Anaximenes of Miletos [585-528 B.C.E.], son of Eurystratos, 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.

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.

"Just as," he said, "our soul, being air, holds us together, so do breath and air encompass the whole world."

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.

It differs in different substances in virtue of its rarefaction and condensation.

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, to stones.

--from the writings of Theophrastus (student of Aristotle)