Protagoras, ‘Man the measure...’ (Sextus Empiricus [a writer of the 2nd century AD] Pyrrhonism 1.216)
“...of all things the measure is man, of things that are that they are, and of things that are not that they are not”--meaning by ‘measure’ (Greek metron) the standard of judgment...By this reckoning he considers only what appears so to a particular person [as ‘reality’], thus introducing relativity... What he says is that matter is in a continuous state of change [and thus] one’s perceptions undergo change with aging or other bodily conditions...Men apprehend different things at different times as their dispositions change... [So much for any fixed reality or moral absolutes]
(Sextus Empiricus, ‘Against the Professors’ 7.65ff.)
[Gorgias] advances 3 propositions: first, that nothing exists; second, if it exists it cannot be known; third, if it can be known (by one man) it certainly cannot be expressed or explained to another.
[By the first propositon, ‘nothing exists’ he may mean, there is no deeper ‘reality’ beyond our perceptions; or it may be largely an exercise in the paradadoxes of language]
He concludes that nothing exists, from the following argument: if something exists it is either the existent or the non-existent or both the existent and non-existent...The non-existent cannot exist, because it would have to exist and not exist at the same time (which is absurd)....[here G seems to be playing on the language of negation, where we say that something is not a certain quality or type, and responding to the idea that the quintessential stuff of reality is ‘unlimited’or undifferented]
Neither does the existent exist [when we say that something is], it is either eternal or originating in time [or both]. ...If it is eternal, it has no beginning, and so it is without limit (or end). And if it is without limit, it must be nowhere. For if it is in some place, that contains it and it is no longer without limit (or boundary). So the unlimited cannot exist anywhere: ...if the existent is eternal, it is without limit (or boundary), [and so] it is nowhere [it cannot be in any place or time], [and so] it does not exist.