Note 55 — Chapter XXIII · PROTAGORAS.
Reading edition
So Serranus (ad 326 E), who has been followed by many later critics. “Quæstio est, Virtusne doceri possit? Quod instituit demonstrare Sophista, sed ineptissimis argumentis et quæ contra seipsum faciant.”
To me this appears the reverse of the truth. But even if it were true, no blame could fall on Protagoras. We should only be warranted in concluding that it suited the scheme of Plato here to make him talk nonsense.