In Plato's The Sophist, a stranger from Elea argues with Theaetetus, a young friend of Socrates, about the definition of "sophist". They come to the conclusion that the sophist imitates the wise man. He gives the appearance of knowledge while actually being a purveyor of falsehood - but this gives rise to a paradox...
Stranger We are really, my dear friend, engaged in a very difficult investigation; for the matter of appearing and seeming, but not being, and of saying things, but not true ones—all this is now and always has been very perplexing. You see, Theaetetus, it is extremely difficult to understand how a man is to say or think that falsehood really exists and in saying this not be involved in contradiction.
Stranger This statement involves the bold assumption that not-being exists, for otherwise falsehood could not come into existence. But the great Parmenides, my boy, from the time when we were children to the end of his life, always protested against this and constantly repeated both in prose and in verse:
Never let this thought prevail, saith he, that not-being is; But keep your mind from this way of investigation.
So that is his testimony, and a reasonable examination of the statement itself would make it most absolutely clear. Let us then consider this matter first, if it's all the same to you.
(Plat. Soph. 236d-237b)
I understand this, simplified, as the step
Speaking what is not ⇒ speaking about non-being (S)
While the context here is resisting a Sophist opponent, who might come up with such arguments, this step (S) still seems like a mere word play to me.
How can (S) be made more plausible? Or can you recommend a commentary that focuses on this problem?