Empedocles of Agrigentum B.c.) regarded reality as eternal, but not as one. It is composed of a multiplicity of ele ments or "roots." Of such "roots" there are four kinds, namely, earth, water, air and fire. These four kinds of elements combine to form the various things there are, and which cease to be when the component elements separate again. In order to account for the combination and separation, Empedocles postulated two en tities or principles, namely, attraction and repulsion, or "love" and "hate." And as a physiologist he compared the relation of these two entities to the "roots" with the relation of air and blood to the living body. His quaint biological speculations are noteworthy for their implicit assumption of the conception of the "survival of the fittest." The Atomists.—The pluralist opposition to the Eleatic con ception of reality as a static, immutable One, culminated in the atomic theory. This was first formulated by Leucippus (?5oo B.c.), who first insisted on the reality of empty space, which used to be regarded as "non-being." But the atomic theory is chiefly associated with the name of his disciple, Democritus, who is commonly described as "the father of physics." Democritus (?460-37o B.c.) is the most important Greek exponent of the atomic theory and the mechanistic method of explanation which is intimately associated with it. According to Democritus atoms and space are the only ultimate realities. The atoms were conceived by him to be of various sizes and shapes. And all the main differences in composite bodies were reduced to differences in the (a) size and shape, (b) position, and (c) ar rangement of the atoms composing them. The atoms may thus be described as the alphabet of the universe, and Aristotle actu ally illustrates these distinctions of Democritus by means of letters of the alphabet. Using Roman instead of Greek letters, the above
distinctions may be illustrated as follows: (a) The letters A and M are different in size and in shape; (b) the letters M and W are different in respect of position; (c) The syllables AM and MA are different in respect of arrangement. The last distinction (c) is clearly reminiscent of the "Figure" or "Form" of the Pytha goreans. It is especially worth noting that the Atomists held that the atoms are naturally endowed with motion, and so avoided the problem, which perplexed all subsequent ages until the time of Newton, as to how passive matter was set in motion. The natural mobility of atoms seemed to require a void or empty space for them to move in. Hence the recognition by the Atomists of the existence of space. Now the atoms flitting about in all directions collide, some of them get hooked together and combine in various ways owing to their various shapes ; then the larger constellations of them enclose other atoms or smaller clusters of them, and so all sorts of things are produced, including whole systems or worlds. Under certain conditions the various clusters break up again into separate atoms. The systematic attempt to explain everything in terms of matter and motion necessitated then, as subsequently, the denial of the ultimate reality of the so-called secondary qualities—colour, sound, taste, smell, etc. So the atomists dismissed them as mere "conventions," that is, as the effects of human intervention, or as merely subjective or illusory appearances. (See DEMOCRITUS.)