(The same principles are laid down in the first, and fourteenth, and fifteenth book of the treatise on Nature; and also in the Great Abridgment.)
“Now, of bodies, some are combinations, and some the elements out of which these combinations are formed. These last are indivisible, and protected from every kind of transformation; otherwise everything would be resolved into non-existence. They exist by their own force, in the midst of the dissolution of the combined bodies, being absolutely full, and as such offering no handle for destruction to take hold of. It follows, therefore, as a matter of absolute necessity, that the principles of things must be corporeal, indivisible elements.
“The universe is infinite. For that which is finite has an extreme, and that which has an extreme is looked at in relation to something else. Consequently, that which has not an extreme, has no boundary; and if it has no boundary, it must be infinite, and not terminated by any limit. The universe then is infinite, both with reference to the quantity of bodies of which it is made up, and to the magnitude of the vacuum; for if the vacuum were infinite, the bodies being finite, then, the bodies would not be able to rest in any place; they would be transported about, scattered across the infinite vacuum for want of any power to steady themselves, or to keep one another in their places by mutual repulsion. If, on the other hand, the vacuum were finite, the bodies being infinite, then the bodies clearly could never be contained in the vacuum.
“Again: the atoms which form the bodies, these full elements from which the combined bodies come, and into which they resolve themselves, assume an incalculable variety of forms, for the numerous differences which the bodies present cannot possibly result from an aggregate of the same forms. Each variety of forms contains an infinity of atoms, but there is not for that reason an infinity of atoms; it is only the number of them which is beyond all calculation.”
(Epicurus adds, a little lower down, that divisibility, ad infinitum, is impossible; for, says he, the only things which change are the qualities; unless, indeed, one wishes to proceed from division to division, till one arrives absolutely at infinite littleness.)
“The atoms are in a continual state of motion.”
(He says, farther on, that they move with an equal rapidity from all eternity, since the vacuum offers no more resistance to the lightest than it does to the heaviest.)
“Among the atoms, some are separated by great distances, others come very near to one another in the formation of combined bodies, or at times are enveloped by others which are combining; but in this latter case they, nevertheless, preserve their own peculiar motion, thanks to the nature of the vacuum, which separates the one from the other, and yet offers them no resistance. The solidity which they possess causes them, while knocking against one another, to re-act the one upon the other; till at last the repeated shocks bring on the dissolution of the combined body; and for all this there is no external cause, the atoms and the vacuum being the only causes.”
(He says, further on, that the atoms have no peculiar quality of their own, except from magnitude and weight. As to colour, he says in the twelfth book of his Principia, that it varies according to the position of the atoms. Moreover, he does not attribute to the atoms any kind of dimensions; and, accordingly, no atom has ever been perceived by the senses; but this expression, if people only recollect what is here said, will by itself offer to the thoughts a sufficient image of the nature of things.)