One of the Dialogues of Plato.
Originally intended to be part of a trilogy of dialogues along with Statesman and Philosopher, this dialogue concerns the nature and attributes of the sophist and his art. By the way, Philosopher either was never written or is no longer extant. This dialogue is unique among Plato's dialogues for it does not have Socrates as the central figure. Rather, the primary facilitator of the dialectic is only referred to as a stranger.
In the first half of the dialogue, the stranger and the young Theaetetus seek to categorize the sophist in the same way one would try to classify a plant or animal. That is, if the subject is of this general class then it is either this or that based on its properties.1 The stranger then defines the two categories, places the subject into one of them, and moves on in the same manner, getting more and more specific each time. The result of this classification is that the two dialecticians find the sophist falling into several categories. He is someone that seeks to purify the soul, to sell a good, to produce a good, to persuade. And when they pursue the sophist down this line of reasoning, they run into trouble. For earlier in their argument, they were able to neatly put other learned folk such as painters and sculptors into neat single categories. The sophist wants to take up several slots.
They latch on to the art of disputation, which the sophist is said to profess. The sophist, in fact, professes to teach the art of disputation of all things. To say such implies that they have knowledge of all things. Obviously, this is impossible so we get the idea that the sophist is something of a con man. In fact they place him in the branch of creative arts having to do with making appearances but not an image, or in their words, fantasy. But Parmenides and other sophists deny that a falsehood can be uttered or professed, because to do so would assert the being of non-being, a thing they deem impossible.
What follows in the dialogue, from this point, is a rather long ontological discussion about what is being and not-being. I won't go into it here because, frankly, I didn't get it. The gist of it was that there is a property of things called being and a class called not being, that a thing may have either the one or the other property. The consequence of the existence of not-being is that falsehood and deceit are possible, for if not-being did not exist all things would be true. In the end, they classify the sophist a deceiver, a professor of fantasies, a juggler of words, and some other unpleasant things. Perhaps someone with better reading skills than I can node the details of the argument better.
Plato also deals with the subject of the sophist and his false teaching in Euthydemus.
1 It is not uncommon in some nature books to try to classify plants, for example, in this manner. Is it woody stemmed or green stemmed? If it is woody stemmed, then does it grow straight up or along the ground? And so forth until the plant is identified.