HERACLEITUS of Ephesus, born about 556 B.C., embodied his system of philosophy in his work _On Nature_. He held that the ground of all phenomena is a physical principle, a living unity, pervading everything, inherent in all things—fire, that is, as he explains, a clear light fluid “self-kindled and self-extinguished.” The world was not created by God, but evolved from the rational intelligence which guides the universe—fire. Fire longs to manifest itself in various forms; from its pure state in heaven it descends, assumes the form of earth, passing in its progress through that of water. Man’s soul is a spark of the divine fire. ANAXAGORAS, born about 499 B.C., was the friend of Pericles and Euripides at Athens. Seeking to explain the world and man by a higher cause than the physical ones of his predecessors, he postulated _nous_—that is, mind, thought, or intelligence. As nothing can come out of nothing, he did not attribute to this _nous_ the creation of the world, but only its order and arrangement. Matter is eternal, but existed as chaos till _nous_ evolved order from the confusion. Baas[349] says his physiological and pathological views may be thus described: “The animal body, by means of a kind of affinity, appropriates to itself from the nutritive supply the portions similar to itself. Males originate in the right, females in the left side of the uterus. Diseases are occasioned by the bile which penetrates into the blood-vessels, the lungs, and the pleura.” He undertook the dissection of animals, remarked the existence in the brain of the lateral ventricles, and was the first to declare that the bile is the cause of acute sickness.[350] DIOGENES OF APOLLONIA, the eminent natural philosopher, lived at Athens about 460 B.C. He was a pupil of Anaximenes, and wrote a work entitled _On Nature_, in which he treated of physical science generally. Aristotle has preserved for us some of the few fragments which remain. The most important is the description of the origin and distribution of the veins, and is inserted in the third book of Aristotle’s _History of Animals_. Diogenes Laertius gives an account of the philosophical teaching of the philosopher: “He maintained that air was the primal element of all things; that there was an infinite number of worlds, and an infinite void; that air, densified and rarefied, produced the different members of the universe; that nothing was produced from nothing, or was reduced to nothing; that the earth was round, supported in the middle, and had received its shape from the whirling round of the warm vapours, and its concretion and hardening from cold.”[351] Diogenes recognised no distinction between mind and matter, yet he considered air possessed intellectual energy.