Anaximenes, the disciple of Thales, pursued his master's inquiries and adopted his method. He also was born in Miletus, but at what time is unknown,--probably 500 B.C. Like Thales, he held to the eternity of matter. Like him, he disbelieved in the existence of anything immaterial, for even a human soul is formed out of matter. He, too, speculated on the origin of the universe, but thought that _air_, not water, was the primal cause. This element seems to be universal. We breathe it; all things are sustained by it. It is Life,--that is, pregnant with vital energy, and capable of infinite transmutations. All things are produced by it; all is again resolved into it; it supports all things; it surrounds the world; it has infinitude; it has eternal motion. Thus did this philosopher reason, comparing the world with our own living existence,--which he took to be air,--an imperishable principle of life. He thus advanced a step beyond Thales, since he regarded the world not after the analogy of an imperfect seed-state, but after that of the highest condition of life,--the human soul. And he attempted to refer to one general law all the transformations of the first simple substance into its successive states, in that the cause of change is the eternal motion of the air. Diogenes of Apollonia, in Crete, one of the disciples of Anaximenes, born 500 B.C., also believed that air was the principle of the universe, but he imputed to it an intellectual energy, yet without recognizing any distinction between mind and matter. He made air and the soul identical. "For," says he, "man and all other animals breathe and live by means of the air, and therein consists their soul." And as it is the primary being from which all is derived, it is necessarily an eternal and imperishable body; but as _soul_ it is also endued with consciousness. Diogenes thus refers the origin of the world to an intelligent being,--to a soul which knows and vivifies. Anaximenes regarded air as having life; Diogenes saw in it also intelligence. Thus philosophy advanced step by step, though still groping in the dark; for the origin of all things, according to Diogenes, must exist in _intelligence_. According to Diogenes Laertius, he said: "It appears to me that he who begins any treatise ought to lay down principles about which there can be no dispute."