Nothing, then, could come out of nothing; all the visible structure of the universe had its origin in the movements of the atoms that constituted it, and conditioned its infinite changes. The atoms, by a useful but perhaps too convenient metaphor, he called the seeds of all things. They were infinite in number, though not infinite in the number, of their shapes. Many atoms were similar to each other, and this similarity formed a basis of union among them, a warp, so to speak, or solid foundation across which the woof of dissimilar atoms played to constitute the differences of things.
[151]
Out of this idea of an eternal eddy or whirl Democritus developed a cosmogony. The lighter atoms he imagined flew to the outmost rim of the eddy, there constituting the heavenly fires and the heavenly aether. The heavier atoms gathered at the centre, forming successively air and water and the solid earth. Not that there was only one such {78} system or world, but rather multitudes of them, all varying one from the other; some without sun or moon, others with greater luminaries than those of our system, others with a greater number. All, however, had necessarily a centre; all as systems were necessarily spherical.
[152]
As regards the atoms he conceived that when they differed in weight this must be in respect of a difference in their essential size. In this he was no doubt combating the notion that the atoms say of lead or gold were in their substance, taking equal quantities, of greater weight than atoms of water or air. The difference of weight in objects depended on the proportion which the atoms in them bore to the amount of empty space which was interlaced with them. On the other hand, a piece of iron was lighter yet harder than a piece of lead of equal size, because of the special way in which the atoms in it were linked together. There were fewer atoms in it, but they were, in consequence of their structure and arrangement, more tightly strung.
[153]
In all this Democritus was with great resolution working out what we may call a strictly mechanical theory of the universe. Even the soul or life-principle in living creatures was simply a structure of the finest and roundest (and therefore most nimble) atoms, with which he compared the extremely attenuated dust particles visible in their never-ending {79} dance in a beam of light passed into a darkened room. This structure of exceeding tenuity and nimbleness was the source of the motion characteristic of living creatures, and provided that elastic counteracting force to the inward-pressing nimble air, whereby were produced the phenomena of respiration. Every object, in fact, whether living or not, kept its form and distinctive existence by its possession in degree of a kind of soul or spirit of resistance in its structure, adequate to counteract the pressure of external forces upon its particles.
[155]
Sensation and perception were forms in which these external forces acted upon the more nimble and lively existences, more particularly on living creatures. For every body was continually sending forth emanations or images resembling itself sufficiently in form and structure to affect perceptive bodies with an apprehension of that form and structure. These images travelled by a process of successive transmission, similar to that by which wave-motions are propagated in water. They were, in other words, not movements of the particles of the objects, which latter must otherwise in time grow less and fade away, but a modification in the arrangement of the particles immediately next the object, which modification reproduced itself in the next following, and so on right through the medium to the perceptive body.
{80}