To the Democritean doctrine, however, Epicurus made a curious addition, to which he himself is said to have attached much importance. The natural course (he said) for all bodies having weight is downwards in a straight line. It struck Epicurus that this being so, the atoms would all travel for ever in parallel lines, and those 'clashings and interminglings' of {217} atoms out of which he conceived all visible forms to be produced, could never occur. He therefore laid it down that the atoms _deviated_ the least little bit from the straight, thus making a world possible. And Epicurus considered that this supposed deviation of the atoms not only made a world possible, but human freedom also. In the deviation, without apparent cause, of the descending atoms, the law of necessity was broken, and there was room on the one hand for man's free will, on the other, for prayer to the gods, and for hope of their interference on our behalf. 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. {218} 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.