The only plausible charge is an accusation of sophistry. This charge is not so different from the actual charge leveled against him by Meletus, namely, that he introduced new gods into the city and corrupted the young; both Aristophanes in the Clouds and the Platonic Socrates in his defense in the Apology (18b) understand the accusation to be that he is somebody who makes the weaker argument appear stronger and thereby corrupts the young, an activity associated with the sophists. The Sophist, with its concern for the correct definition of the sophist, can then be understood as a methodical attempt to figure out whether there is anything to the charge.
To be sure, the Eleatic Stranger never makes this accusation. He and Theaetetus simply develop various definitions of the sophist, never explicitly connecting them with Socrates. The fifth definition of sophistry, however, beginning at 226a and ending at 231b8, is, as scholars have long recognized, alarmingly close to Socrates' own practice as described in the "Socratic" dialogues. The sophist that the Stranger and Theaetetus describe engage in the refutation of the ignorance that does not know it is ignorance, as a form of education, which certainly seems to be much like what Socrates does in many "early" Platonic dialogues. Here's the Stranger's final summary of their definition (231b, Fowler translation in Perseus):
... let it be agreed that part of the discriminating art is purification, and as part of purification let that which is concerned with the soul be separated off, and as part of this, instruction, and as part of instruction, education; and let us agree that the cross-questioning of empty conceit of wisdom, which has come to light in our present discussion, is nothing else than the true-born art of sophistry.Is the Stranger indicting Socrates by this definition of sophistry? Though Theaetetus is ready to describe the master of the cathartic art as a sophist, the Stranger seems uncertain (230e-231b):
Well then, who are those who practise this art?
[231a] I am afraid to say the sophists.
Lest we grant them too high a meed of honor.
But the description you have just given is very like someone of that sort.
Yes, and a wolf is very like a dog, the wildest like the tamest of animals. But the cautious man must be especially on his guard in the matter of resemblances, for they are very slippery things. However, let us agree that they are the sophists; for I think the strife will not be about petty discriminations [231b] when people are sufficiently on their guard.
It is not clear here whether the sophist they have described (and hence Socratic practice) is the wolf or the dog in this analogy, and hence whether the Stranger is indicting or absolving Socrates.
Scholars disagree as well: some feel compelled to deny that the image of the “cathartic” sophist really applies to Socrates at all (Bluck 1975, pp. 40-46; Diès 1925, p. 272; Kerferd 1954), even if the similarities must be acknowledged as part of the philosophic “trial” of Socrates (Friedländer 1964, p. III.237); others argue that Plato himself is in doubt (Campbell 1867, p. li); yet others deny that the Stranger “disvalues” this sort of sophistry, though they acknowledge that it is a sort of sophistry that needs to be superseded, perhaps by by the positive method of the latter Plato (Cornford 1935; Dorter 1994, pp. 131-134; Heidegger 1997/1924-2, p. 263; Kerferd 1954), pp. 85-97); some see here a kind of (ambiguous) indictment of Socrates (Benardete 1984, p. II.99; Howland 1998, pp. 203-206; Notomi 1999, section 2.3, pp. 64-68); finally others see here an association between Socrates and sophistry, but think that this implies that sophistry is necessary and universal, and so see no real "indictment" of Socrates (Wolff 1991, esp. p. 51). The very variety of scholarly reactions to the question testifies to the ambiguity of the passage and the stakes of the issue: scholars feel compelled to take sides on the matter, to decide whether Socrates should be indicted as a sophist or not, and if not, why not.
If Socrates appears as a sophist, it should be noted that the Eleatic Stranger does not escape suspicion either (Lassègue 1991, in a weak sense; Scodel 1987; Tejera 1999, chapters 10 and 11). As Notomi (1999, p. 72) notes, it is only Theodorus “who introduced and regarded him as a philosopher;” and certain features of the definitions of the sophist (primarily the second, the itinerant merchant of learning, and the last, the human producer of distorted images of wisdom [such as the Statesman’s myth] in an intelligible medium, ironically, in private with short speeches that can produce contradiction) seem applicable to the Stranger even more than to Socrates. One could also add that at one point the Stranger does imitate a sophist fairly explicitly (239e1-240c3), a passage commented on by Campbell (1867, p. xxii): “The Eleatic Stranger is like the Sophist he describes, whose “sense is shut” to everything but the dry light of reason.”
This confusion of appearances is simply a restatement of the problem posed by Socrates at the very beginning of the dialogue (216c-d): the philosopher sometimes appears as a statesman, sometimes as a sophist, sometimes as a madman, and are therefore hard to make out clearly. Plato seems to leave it as an exercise for the reader to distinguish Socrates from the sophist or else to indict him and find a new philosophical path; we are invited to take sides. It is never absolutely clear whether Socrates is a sophist; at any rate, he is perilously similar to the sophist, like a dog to a wolf, and pinpointing the essential difference is not easy from a distance.
In order to see the difference between them we have to understand images - what they are and how they work; and so the dialogue moves on to a thorough consideration of images. More on images this week...