Diogenes of Apollonia, in Crete, one of his disciples, born B.C. 460, 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. [Footnote: Diog. Laert., ii. 3; Bayle, _Dict. Hist. et Crit._] 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." [Footnote: Ritter, b. iii. c. 3.] 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_. [Sidenote: Heraclitus--Fire the principle of life.] Heraclitus of Ephesus, classed by Ritter among the Ionian philosophers, was born B.C. 503. Like others of his school, he sought a physical ground for all phenomena. The elemental principle he regarded as _fire_, since all things are convertible into it. In one of its modifications, this fire, or fluid, self-kindled, permeating every thing as the soul or principle of life, is endowed with intelligence and powers of ceaseless activity. "If Anaximenes discovered that he had within him a power and principle which ruled over all the acts and functions of his bodily frame, Heraclitus found that there was life within him which he could not call his own, and yet it was, in the very highest sense, _himself_, so that without it he would have been a poor, helpless, isolated creature; a universal life which connected him with his fellow-men,--with the absolute source and original fountain of life." [Footnote: Maurice, _Moral and Metaph. Phil._] "He proclaimed the absolute vitality of nature, the endless change of matter, the mutability and perishability of all individual things in contrast with the eternal Being--the supreme harmony which rules over all." [Footnote: Lewes, _Biog. Hist. of Phil._] To trace the divine energy of life in all things was the general problem of his philosophy, and this spirit was akin to the pantheism of the East. But he was one of the greatest speculative intellects that preceded Plato, and of all the physical theorists arrived nearest to spiritual truth. He taught the germs of what was afterwards more completely developed. "From his theory of perpetual fluxion Plato derived the necessity of seeking a stable basis for the universal system in his world of ideas." [Footnote: Archer Butler, series i. lect. v.; Hegel, _Gesch. D. Phil._, i. p. 334.]