1. In the teaching of Empedocles we have had the first distinct outlines of a doctrine of Dualism. Not only does he assume, over and above the causa materialis of Nature, certain motive and formative forces, but he further sets the One, the force of Love, above Matter, since he assigns to it the attributes of the Deity. In contrast with Empedocles, the Atomists, Leucippus and Democritus, denied all immaterial force; they admitted matter and nothing more. Moreover, their notion of matter was not that of the earlier Ionians, who represented it as a principle at once material and dynamical; to them it was material in the strictest sense of the term. Their attempt to explain the origin of things from such a principle necessarily brought them to a lower system than hylozoism, to pure Materialism and Casualism (i.e., the doctrine which attributes the origin of things to chance, casus).
2. Little is known as to the time at which Leucippus lived, and as to the incidents of his life. It is not even known with certainty whether he was an inhabitant of Abdera, of Miletus, or of Elea. His pupil Democritus was born (about B.C. 400) at Abdera. The desire for knowledge, it is said, led Democritus to make long journeys into Egypt and the East. He was the author of many works, among which the Megas Diakosmos was the most celebrated.
3. According to Aristotle (Met. 1, 4) the Atomists assumed as the fundamental principle of all things the Empty (kenon) and the Full (plêres), characterising the former as non-being (mê on), the latter as being (on), whence their dictum that non-being exists as well as being. Closer inquiry into the nature of these conceptions shows us that the Empty is another expression for the notion of boundless space, the Full another expression for an infinite number of atoms contained within it. These two principles of things the Atomists held to be sufficient for the explanation of the origin of the Universe.
To establish the existence of empty space Democritus adduced the following proofs (Arist. Phys., 4, 6): -- 1st. Motion requires a vacuum, for what is full cannot receive anything into itself. 2nd. Rarefaction and condensation are possible only on the supposition that empty interstices exist in bodies. 3rd. Growth depends on the penetration of nutriment into the empty spaces of the organic body. 4th. A vessel containing ashes does not admit a quantity of water less by a volume equal to the space occupied by the ashes, the one must therefore in part enter into the empty interstices of the other.
4. The Atomists understand by atoms minute indivisible particles out of which all things corporeal are made. The atoms are all alike in specific character, but they differ in shape or conformation (schema). They differ also in size; the weight of each atom corresponds to its size. It is useless to ask any question as to the origin of the atoms -- they are eternal and therefore are not dependent on a cause.
5. In explaining the origin of corporeal things from these atoms, the Atomists suppose them endowed with a primordial eternal motion. If we ask what is the cause of this motion we are answered that we must not look for any cause above themselves. Like the atoms themselves, their movement has no cause, their motion is a necessary condition of their existence, and for this necessity no ulterior reason can be assigned.
6. This movement it is which determines the existence of the world. Owing to the difference in weight the heavier atoms fall, and the lighter atoms rise, and the collisions of the atoms also give rise to lateral movements. In this way an eddying motion (dinê) is produced, which, extending ever further and further, brings about the formation of worlds. In the revolution many atoms unite permanently together, and in such wise too, that like is joined to like, a vacuum is created in the interstices of the aggregate thus formed, and so larger composite bodies, and finally whole worlds come to exist. In all this process there is neither Purpose nor Law, mere Chance governs the whole (Casualism). Worlds without number come into being in this fashion.
7. The differences between things in nature are explained by the Atomists on the same principles. In the countless worlds which come into existence, different combinations of atoms are formed, and these assume different shapes; the round, the angular, the hook-shaped atoms arrange themselves in combination to form various kinds of surfaces. These surfaces, affecting our organs of sense, occasion perceptions, which we style sensible qualities of things, but which in reality are nothing more than an arrangement of figures. The qualities of a given body are merely something corresponding to the figures which go to make up their surfaces.
8. "The earth was at first in motion, and so continued as long as it was small and light; gradually, however, it was brought to rest. Organic structures were evolved from the moistened earth, and are all alike animated. The soul consists of subtle, smooth, round atoms, which are also the atoms that constitute fire. Atoms of this kind are scattered through the whole body, but they exercise different functions in different organs. The brain is the region of thought, the heart of anger, the liver of desire. At every inhalation we draw in physical atoms out of the air, at every exhalation we give them out again; and life lasts as long as this process continues." The soul is not immortal. But it is, nevertheless, the noblest part of man; he who seeks the good of the soul seeks what is divine; who seeks the good of the body -- the covering or tent of the soul, seeks what is human.
9. The perceptions of sense are due to an efflux of atoms from the objects perceived. These atoms form themselves into images (eidôla), which strike the organs of sense, and find entrance through them. The knowledge which rests on sense-perception alone is an obscure knowledge (skotiê), from which we must carefully distinguish genuine knowledge (gnosiê), which is the fruit of inquiry by the understanding.
10. The supreme good is Happiness. "This is attained by the avoidance of extremes, and the observance of moderation, not by any external good." A necessary means to the same end is a right insight into the nature of things. Knowledge seems the highest contentment. Our dispositions, not our acts, determine our moral character.
11. Among the many supporters of the atomist theories of Leucippus and Democritus, Metrodorus of Chios and his pupils, Anaxarchus and Hippocrates, are specially worthy of mention. These philosophers seem to have emphasised and developed the elements of scepticism involved in Democritus' theory of sensuous perception.
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