With Democritus Epicurus agreed in holding that there is no other form of reality except that of bodily reality. Every substance, he says in the words of the Stoics, must affect others, and be affected by them; and whatever affects others or is itself affected, is corporeal. Corporeal substance is, therefore, the only kind of substance. [1156] The various qualities of things, essential as well as accidental qualities, are accordingly not incorporeal existences, but simply chance modes of body, the former being called by Epicurus συμβεβηκότα, the latter συμπτώματα. [1157] But a second something is necessary besides corporeal substance in order to explain phenomena, viz. empty space. That empty space exists is proved by the differences of weight in bodies. For what else could be the cause of this difference? [1158] It is proved still more conclusively by motion, motion being impossible without empty space. [1159] Mind as a moving cause, however, seems to Epicurus altogether superfluous. Everything that exists consists of bodies and empty space, and there is no third thing. [1160] Democritus had resolved the two conceptions of body and empty space into the conceptions of being and not being. True to his position, Epicurus dispensed with this speculative basis, and clinging to the ordinary notions of empty space, and of a material filling space, [1161] he simply proves these notions by the qualities of phenomena. For this very reason Democritus’s division of body into innumerable primary particles or atoms appeared to him most necessary. All bodies known to us by sensation are composed of parts. [1162] If the process of division were infinitely continued, all things would ultimately be resolved into the non-existent—in this Epicurus and Democritus agree;—and conversely all things must have been formed out of the non-existent, in defiance of the first principle of natural science that nothing can come from nothing, and that nothing can be resolved into nothing. [1163] Hence, we must conclude that the primary component parts of things can neither have come into existence nor cease to exist, nor yet be changed in their nature. [1164] These primary bodies contain no empty space in themselves, and hence can neither be divided nor destroyed, nor be changed in any way. [1165] They are so small that they do not impress the senses, and as a matter of fact we do not see them. Nevertheless they must not be regarded as mathematical atoms, the name atoms being assigned to them only because their bodily structure will not admit of division. [1166] They have neither colour, warmth, smell, nor any other property; properties belong only to distinct materials; [1167] and for this reason they must not be sought in the four elements, all of which, as experience shows, come into being and pass away. [1168] They possess only the universal qualities of all corporeal things, viz. shape, size, and weight. [1169]