I do not usually quote from Wikipedia, but here it, well, fits..
Pyrrho (/ˈpɪroʊ/; Greek: Πύρρων Pyrrōn, c. 360 BC – c. 270 BC), a Greek philosopher of Classical Antiquity, was a student of Eastern philosophy and is credited as being the first Greek skeptic philosopher and the inspiration for the school known as Pyrrhonism, founded by Aenesidemus in the 1st century BC.
Pyrrho was reported, by his skeptic followers in later eras, to be the first philosopher who argued, directly and thoroughly, about our lack of ability to know reality as it is. This should be distinguished from:
The common view, among ancient Greek philosophers, that we usually do not in fact know the true reality. By that it was not meant that reality was unknowable, just that it was usually not known as it is.
Heraclitus's (~535-475 BC) view that reality itself lacked identity ("you cannot step into the same river twice") so that it could not be known, because there was nothing to know. And not because our cognition was somehow limited.
Skeptical views among some of the sophists (known to us especially from Plato's dialogues), notably Protagoras (~490-420 BC). Protagoras is remembered especially for the following saying:
Man is the measure of all things: of the things that are, that they are, of the things that are not, that they are not.
Apparently not enough remained, to assess how far Protagoras and others went in purely philosophical skepticism. In general, anyway, unlike the later school of skeptics, the sophists were not inclined to immerse themselves in logical skeptical arguments. Instead, they preferred rhetoric over logic, and they taught young men how to use rhetoric for practical success, in private and public life.