The dates of the last philosopher of the Milesian school, Anaximenes are very uncertain. Of his life nothing is known. He was certainly subsequent to Anaximander, and he could not flourish after 494 B.C., since in that year Persians destroyed Miletus. But according to Apollodorus, quoted by Diogenes, Anaximenes flourished from about 585 to 524 B.C.
According to Anaximenes, the air is the primal substance from which all other substances such as fire, water, earth etc., are derived. For him, states of substance (solid, liquid, gas) are merely the product of rarefaction and condensation process. The soul is air, fire is rarefied air; when condensed, and air becomes water, if further condensed water becomes earth and finally becomes stone. Anazimenes says that air is the first principle of all things, and that it is infinite in quantity but is defined by its qualities; and all things are gererated by a certain condesation or rarefaction of air. He thought that by the compression of the air the earth was formed and the shape of the earth is like a round table, and that air covers everything. He maintained that earth rests on air; and the sun and the moon and the rest of the starts were formed from earth. He said that the sun is earth beacause of its motion, and it has the proper amount of heat. Although he was not quite so interesting as compared to Anaximander, it is important to note he had an important influence on Pythagoras.
Briefly, Anaximenes adopted a version of Thales hypothesis, but he changed the original principle from water to air.