From the Beginning to Plato

We cannot assert the existence of “what is not” without contradicting ourselves. Parmenides of Elea came up with this conversation‐stopping argument about two‐and‐a‐half millenia ago, and used it in showing that, contrary to how we “erring mortals” tend to interpret the evidence of our senses, reality is necessarily single, unchanging, and indivisible (On Nature). Protagoras of Abdera (c. 490‐420) seems to have thought that the world is just as it appears to the senses of mortals; his treatise On Truth, of which only fragments survive, begins with the famous words: “Of all things man is the measure”....