According to a fragment of Porphyry (410 F Smith = 80 B 2 DK), containing a dialogue on the theme of plagiarism, Plato made use of the same arguments as Protagoras’ Περὶ τοῦ ὄντος against monistic thinkers, most likely the Eleatics. My paper aims to analyse Porphyry’s testimony to assess some aspects of Protagoras’ reflection on being through a comparison with parallel sources, in particular Plato’s dialogues (Theaetetus, Euthydemus, Sophist, Parmenides). I conclude that it is plausible to suppose that, within (...) the framework of a more general reflection on ὀρθοέπεια, Protagoras viewed the notion of being in Περὶ τοῦ ὄντος from a linguistic perspective. (shrink)