Very little survives of Protagoras’ Alêtheia or ‘Truth’. It was almost certainly not a well-developed philosophical treatise on the nature of truth, but was a display-piece, meant to showcase Protagoras’ skill in argumentation—especially in arguing against philosophers like Parmenides. The book probably did not contain much more than a few arguments for the measure doctrine, the claim that things are for each as they appear to each, and it is unlikely that Protagoras elaborated or developed the philosophical implications of the measure doctrine. The central idea Protagoras proposed was the idea that human beings are measures or ‘criteria’ of what is true.
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