It may be worth while summarising the proofs which Lucretius in his great poem, professedly following in the footsteps of Epicurus, adduces for these various doctrines.
Epicurus' first dogma is, 'Nothing proceeds from nothing,' that is, every material object has some matter previously existing exactly equal in quantity to it, out of which it was made. To prove this Lucretius appeals to the order of nature as seen in the seasons, in the phenomena of growth, in the fixed relations which exist between life and its environment as regards what is helpful or harmful, in the limitation of size and of faculties in the several species and the fixity of the characteristics generally in each, in the possibilities of cultivation and improvement of species within certain limits and under certain conditions.
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To prove his second position, 'Nothing passes into nothing,' Lucretius points out to begin with that there is a law even in destruction; force is required to dissolve or dismember anything; were it otherwise the world would have disappeared long ago. Moreover, he points out that it is from the elements set free by decay and death that new things are built up; there is no waste, no visible lessening of the resources of nature, whether in the generations of living things, in the flow of streams and the fulness of ocean, or in the eternal stars. Were it not so, infinite time past would have exhausted all the matter in the universe, but Nature is clearly immortal. Moreover, there is a correspondence between the structure of bodies and the forces necessary to their destruction. Finally, apparent violations of the law, when carefully examined, only tend to confirm it. The rains no doubt disappear, but it is that their particles may reappear in the juices of the crops and the trees and the beasts which feed on them.
Nor need we be surprised at the doctrine that the atoms, so all-powerful in the formation of things, are themselves invisible. The same is true of the forest-rending blasts, the 'viewless winds' which lash the waves and overwhelm great fleets. There are odours also that float unseen upon the air; there are heat, and cold, and voices. There is the process of evaporation, whereby we know that the water has gone, {219} yet cannot see its vapour departing. There is the gradual invisible detrition of rings upon the finger, of stones hollowed out by dripping water, of the ploughshare in the field, and the flags upon the streets, and the brazen statues of the gods whose fingers men kiss as they pass the gates, and the rocks that the salt sea-brine eats into along the shore.
That there is Empty Space or Void he proves by all the varied motions on land and sea which we behold; by the porosity even of hardest things, as we see in dripping caves. There is the food also which disperses itself throughout the body, in trees and cattle. Voices pass through closed doors, frost can pierce even to the bones. Things equal in size vary in weight; a lump of wool has more of void in it than a lump of lead. So much for Lucretius.
For abstract theories on physics, except as an adjunct and support to his moral conceptions, Epicurus seems to have had very little inclination. He thus speaks of the visible universe or Cosmos. [373] The Cosmos is a sort of skyey enclosure, which holds within it the stars, the earth, and all visible things. It is cut off from the infinite by a wall of division which may be either rare or dense, in motion or at rest, round or three-cornered or any other form. That there is such a wall of division is quite admissible, for no object of which we have observation is without its limit. Were this wall of division to {220} break, everything contained within it would tumble out. We may conceive that there are an infinite number of such Cosmic systems, with inter-cosmic intervals throughout the infinity of space.
He is very disinclined to assume that similar phenomena, e.g. eclipses of the sun or moon, always have the same cause. The various accidental implications and interminglings of the atoms may produce the same effect in various ways. In fact Epicurus has the same impatience of theoretical physics as of theoretical philosophy. He is a 'practical man.'
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He is getting nearer his object when he comes to the nature of the soul. The soul, like everything else, is composed of atoms, extremely delicate and fine. It very much resembles the breath, with a mixture of heat thrown in, sometimes coming nearer in nature to the first, sometimes to the second. Owing to the delicacy of its composition it is extremely subject to variation, as we see in its passions and liability to emotion, its phases of thought and the varied experiences without which we cannot live. It is, moreover, the chief cause of sensation being possible for us. Not that it could of itself have had sensation, without the enwrapping support of the rest of the structure. The rest of the structure, in fact, having prepared this chief cause, gets from it a share of what comes to it, but not a share of all which the soul has.