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’ Π ερὶ 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.