  Epicurus is an atomist, and in accordance with his atomism he takes the soul, like everything else that there is except for the void, to be ultimately composed of atoms. Our sources are somewhat unclear as to exactly which kinds of materials he took to be involved in the composition of soul. It is very probable, though, that in addition to some relatively familiar materials — such as fire-like and wind-like stuffs, or rather the atoms making up such stuffs — the soul, on Epicurus' view, also includes, in fact as a key ingredient, atoms of a nameless kind of substance, which is responsible for sense-perception. Thus it seems that while he thought he could explain phenomena such as the heat or warmth of a living organism, as well as its movement and rest, by appealing to relatively familiar materials and their relatively familiar properties, he did feel the need to introduce a mysterious additional kind of substance so as to be able to explain sense-perception, apparently on the grounds that “sense-perception is found in none of the named elements” (L&amp;S 14C). It is worth noting that it is specifically with regard to sense-perception that Epicurus thinks the introduction of a further, nameless kind of substance is called for, rather than, for instance, with regard to intellectual cognition. What this suggests, and what in fact we have independent reason to think, is that on Epicurus' view, once one is in a position adequately to explain sense-perception, one will then also be in a position to work out an explanation of intellectual cognition, by appropriately extending the explanation of sense-perception. Let us consider briefly how such extension might work. 