Democritus that the universe consists of indivisible atoms bounding together into compound structures, but wished to escape the conclusion of free will and sense reliability as illusional."Sweet exists by convention,Bitter by convention,Colour by convention;Atoms and void [alone] exist in reality- Democritus
Democritus famously said our senses fool us into attributing properties to objects which, as mere aggregations of atoms, cannot have those properties. An orange for example may taste sweet to one person but bitter to another because an orange itself is neither bitter nor sweet: the taste is just the causal interaction between an orange and our sense organs. This realization led Democritus into a despairing subjective skepticism, believing all knowledge, including his own atomic theory, is suspect as the very senses we rely upon to acquire such knowledge are themselves unreliable.
Epictetus thought this both wrong and impractical. Yes, an orange is not intrinsically sweet or bitter but to conclude sweetness is nonexistent and that our senses are unreliable is to misunderstand the nature of our atomic universe. Rather an orange really is sweet if it has the atomic properties to generate 'sweetness' when interacting with our taste buds. And an orange really is bitter if the atomic interaction causes bitterness. Democritus's mistake was assuming properties must be intrinsic in order for us to gain knowledge: that is, to state something is X, means it must be X at all times. But in a material world of atomic interactions we should expect relational properties which vary depending on circumstances and this relativity is not misleading as it's generated by the underlying reality of interacting atoms. Sweetness and bitterness do not exist by mere convention.
The second casualty of Democritus was free will. If our minds too are atomic aggravations then our decisions and actions are causally necessarily and our will cannot be free. Unfortunately much of Epictetus original writings are lost including his defense of free will and so we are forced to rely upon much later philosophers like Lucretius.
According to Lucretius, Epicureans believed atoms fall straight downward through an infinite void but would randomly 'swerve' causing a causal chain reaction which bonded atoms together into compound bodies. As this atomic swerve is random, it therefore breaks any preexisting causal chains, freeing us from our predetermined destiny.
But this argument does not support free will: even if a atomic serve did occur, we are still determined by interacting atoms and unable to freely choose. It's worth noting however scholars can only agree that Epictetus opposed determinism; his actual position remains in doubt.