In expanding their views on the origin of the world, the Stoics begin with the doctrine of the four elements, [448] a doctrine which, since the time of Aristotle and Plato, was the one universally accepted. They even refer this doctrine to Heraclitus, desiring, above all things, to follow his teaching in natural science. [449] On a previous occasion, the order and the stages have been pointed out, according to which primary fire developed into the several elements in the formation of the world. [450] In the same order, these elements now go over one into the other. Yet, in this constant transformation of materials, in the perpetual change of form to which primary matter is subject, in this flux of all its parts, the unity of the whole still remains untouched. [451] The distinctive characteristic of fire is heat; that of air is cold; that of water, moisture; dryness that of the earth. [452] These essential qualities, however, are not always found in the elements to which they belong in a pure state, [453] and hence every element has several forms and varieties. [454] Among the four essential qualities of the elements, Aristotle had already singled out two, viz. heat and cold, as the active ones, calling dryness and moisture the passive ones. The Stoics do the same, only more avowedly. They consider the two elements to which these qualities properly belong to be the seat of all active force, and distinguish them from the other two elements, as the soul is distinguished from the body. [455] In their materialistic system, the finer materials, as opposed to the coarser, occupy the place of incorporeal forces. The relative density of the elements also determines their place in the universe. Fire and air are light; water and earth are heavy. Fire and air move away from the centre of the universe; [456] water and earth are drawn towards it; [457] and thus, from above to below—or, what is the same thing, from without to within—the four layers of fire, air, water, and earth are formed. [458] The fire on the circumference goes by the name of Ether. [459] Its most remote portion was called by Zeno Heaven; [460] and it differs from earthly fire not only by its greater purity, [461] but also because the motion of earthly fire is in a straight line, whereas the motion of the Ether is circular. [462] Because of this difference of motion, Aristotle supposed a radical difference to exist between these two kinds of fire, but the Stoics did not feel it necessary to admit such a difference. [463] They could always maintain that, when beyond the limits of its proper locality, fire tried to return to it as quickly as possible, whereas within those limits it moved in the form of a circle. [C. The universe.]