While Epicurus does make significant changes to Democritean atomism by adding weight and the swerve as causes of atomic motion, his ontology at the level of atoms is basically the same: the ultimate constituents of the universe are void, which is simply empty space, and atoms, which are extended bits of matter, eternal and changeless except in their locations. We infer that these entities exist on the basis of our perception of a world of changeable, temporary objects, objects that, unlike the atoms, have properties such as being sweet, hot and red.

Democritus, however, famously denies that these sorts of properties exist in objects. Orange juice may appear to be sweet, but in reality it is no more sweet than it is sour. So our senses systemically mislead us, representing objects as having properties they do not really have, and this makes knowledge difficult, if not impossible, to attain. Epicurus believes that these conclusions are unacceptable, in part because this scepticism would have devastating practical consequences. He needs to reaffirm the reality of these sensible qualities, even while accepting Democritus' basic atomist ontology and theory of perception.

Democritus and the unreality of sensible qualities

After establishing that atoms and void exist, Democritus gives a detailed account of how the perceptual qualities associated with each sense arise as a result of the causal interaction of atoms with the sense-organs. For instance, the taste “bitter” is explained as a result of sharp atoms tearing the tissue of the tongue, “sweet” as the soothing action of round and fairly large atoms on the tongue. The Epicureans largely accept Democritus’ account.