ANAXIMANDER, born B.C. 610, is said to have been a pupil of Thales. He taught that a single determinate substance having a middle nature between water and air was the infinite, everlasting, and divine, though not intelligent material from which all things had their origin. This he called the ἄπειρον, the chaos. All substances were derived thence by the conflict of heat and cold and the electric affinities of the particles. The atomic theory is foreshadowed here.
ANAXIMENES was the friend of Thales and Anaximander, and all three were born at Miletus. He considered that air was the first cause of all things, or primary condition of matter; all finite things were formed from the infinite air by compression or rarefaction produced by eternal motion. Heat and cold are produced by the varying density of the primal element. He held the eternity of matter like his brother philosophers, and believed that the soul itself is merely a form of air. He held no Divine Author of the Universe, motion being a necessary law of the universe, and with motion and air he required nothing else for the constitution of all things.
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]