The physical theory of Democritus also yielded a rich scientific _explanation of the historical evolution of the universe_. The universe, according to Democritus,――following the teaching of Leucippus,――consists of two parts: the Plenum or self-moving, qualitatively similar atoms; and the Void or empty space, in which the atoms move. The Plenum, or the atoms, is Being; the Void is not-Being. The atoms differ only in form and size;[24] they are infinite in number and therefore are of an infinite number of forms and sizes; they are imperceptibly small. The perceptible qualities do not belong to them, but to their motions. Motion is an irreducible function of atoms, and each atom, lawless in itself, is in flight through space. An aggregation of atoms arises when the atoms meet in their cosmic flight. The shock causes a vortex which draws more atoms into itself. Like atoms are drawn together, and the heavy atoms press the fine fire-atoms to the periphery. Thus innumerable worlds are formed, for any place of the meeting of several atoms can be the beginning of a new world. Sometimes small worlds are drawn into the vortices of large worlds, and sometimes large worlds disintegrate in fatal collisions. The worlds are therefore endless and in endless succession. The whole swings in space like a ball; the rim of the whole consists of compact atoms; the centre is filled with air. To much further length than we can go here Democritus developed a theoretical description of cosmic evolution upon the principle of mechanical necessity――and the description is almost modern. =The Materialistic Psychology of Democritus.= It is easy to understand an explanation of the physical universe as atoms in motion; for our modern scientific theories of nature are set in these terms, even if we have transformed the Democritan static atom into a dynamic entity. It is rather more interesting to follow such a materialist as Democritus in his extension of the materialistic principle over upon the realm of the mental life. In the first place, Democritus conceives man to be part and parcel of the world of atoms. Man is composed of all kinds of atoms. His body consists of earth, water, and air atoms. His mind is made up of fire atoms, which differ from the others in being the finest, smoothest, and most mobile. On this account the fire atoms are the most perfect of all. Psychical activity is the motion of fire atoms. They are scattered throughout the universe, and wherever they are, there is life. They are in plants and animals as well as in man. There is a larger collection of them in man, and this shows his superiority over other living things. In man there is a fire atom between every two other atoms, and the whole is held together by breathing. The different forms of mental activity are simply different forms of atomic motion.