Anaxagoras, the most famous of the Ionian philosophers, was born 500 B.C., 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, under which the Attic philosophers sought to explain Nature, by regarding as the cause of all things, not _matter_ in its different elements, but rather _mind_, thought, intelligence, which both knows and acts,--a grand conception, unrivalled in ancient speculation. This explanation of material phenomena by intellectual causes was the peculiar merit of Anaxagoras, 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,"--the complaint, not of a sceptic, but of a man overwhelmed with the sense of his incapacity to solve the problems which arose before his active mind. 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.