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.

[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 asfire, 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.]

Anaxagoras, the most famous of the Ionian philosophers, was born B.C. 500, and belonged to a rich and noble family. Regarding philosophy as the noblest pursuit of earth, he abandoned his inheritance for the study of nature. He went to Athens in the most brilliant period of her history, and had Pericles, Euripides, and Socrates for pupils. He taught that the great moving force of nature was intellect [Greek: nous]. Intelligence was the cause of the world and of order, and mind was the principle of motion; yet this intelligence was not a moral intelligence, but simply the primum mobile - the all-knowing motive force by which the order of nature is effected. He thus laid the foundation of a new system which, under the Attic philosophers, sought to explain nature, not by regarding matter in its different forms, as the cause of all things, but rather mind, thought, intelligence, which both knows and acts - a grand conception unrivaled in ancient speculation. This explanation of material phenomena by intellectual causes was his peculiar merit, and places him in a very high rank among the thinkers of the world. Moreover, he recognized the reason as the only faculty by which we become cognizant of truth, the senses being too weak to discover the real component particles of things. Like all the great inquirers, he was impressed with the limited degree of positive knowledge, compared with what there is to be learned. "Nothing," says he, "can be known; nothing is certain; sense is limited, intellect is weak, life is short" [Footnote: Cicero, Qu. Ac., i. 12.] - the complaint, not of a skeptic, but of a man overwhelmed with the sense of his incapacity to solve the problems which arose before his active mind. [Footnote: Lucret., lib. i. 834-875.] Anaxagoras thought that this spirit [Greek: Nous] gave to all those material atoms, which, in the beginning of the world, lay in disorder, the impulse by which they took the forms of individual things, and that this impulse was given in a circular direction. Hence that the sun, moon, and stars, and even the air, are constantly moving in a circle. [Footnote: Muller, Hist. Lit. of Greece, chap. xvii.]

[Anaximander thought that the Infinite is the origin of things.]