10. The Atomism of the School of Abdera. Democritus. -- LEUCIPPUS is the founder, but DEMOCRITUS (about 460-370) is the accredited representative of the atomist school. He himself tells us that in his early youth he knew Anaxagoras as an old man. Democritus was a man of science as well as a philosopher; he travelled in search of knowledge through Egypt and possibly as far as Babylonia. At Abdera, his birthplace, he knew Leucippus and followed his lectures.

Here are the fundamental principles of the teaching of Democritus: (1) Matter is composed of an unlimited multitude of tiny corpuscles qualitatively homogeneous but differing in shape and size: these are the atoms (atoma). The atom is of itself inert, eternal, indivisible, solid, continuous; it encloses no vacant space within it, for vacuum is the principle of divisibility (Parmenides). Not merely are the formation and dissolution of bodies explained by the accumulation and separation of atoms, but all phenomena are reduced to more or less transitory atomic structures.

(2) Democritus does not accept the fiction of love and hatred as an explanation of motion; he attributes this phenomenon to the action of weight and the existence of vacuum or empty space. This latter is essential for motion: if all space were full of matter, as Parmenides had taught, the atoms would be all packed together and no change would be possible (6). On the other hand, admit an interatomic vacant space and the atoms are free to move if there be any agency to move them. Weight draws the atoms downwards and thus sets them in motion; and since they are of unequal sizes, the larger, which are also the heavier, strike the smaller ones and impress on them a non-vertical motion: the shocks due to those impulses provoke a constant eddying movement and give rise to the formation of atomic combinations or worlds. Motion being eternal, space being without limits, and the multitude of the atoms being infinite, there are in existence innumerable worlds.

Democritus applies those general principles to the world we live in, and especially to man himself. His psychology is without any special psychological method; it is a mere chapter of his physical atomism. Man's soul, like his body, is an assemblage of atoms of a lighter and subtler order. Sensation and thought are only vibrations of atoms; they are stirred up in us by material emanations from outer objects, emanations which pass through the intervening space and enter our organs: this is the famous theory of the atomic images or species (eidola). These same images are fertile seeds of scepticism, for the medium modifies the material emanations, which are accordingly incapable of giving us a knowledge of things as they are. The philosophy of Democritus is a clear and emphatic assertion of materialistic atomism.