Malcolm Schofield in Plato: Political Philosophy is so confident that the 'mouthpiece' view is incorrect, that he uses this as evidence against the authenticity of the Seventh Letter.
The author of the Seventh Letter writes:
I was compelled to say ... that the classes of mankind will find no release from their troubles until either the class of those who engage properly and truly in philosophy take on political positions, or the class of those who wield power in the cities engage in real philosophy by some dispensation of divine providence (326A-B).Schofield points out that the passage is written in such a way as to intend to refer to the Republic. Whence his objection:
But in the Republic it was not Plato speaking in his authorial person who said he was compelled to argue for truly philosophical rulers. It was Socrates--Plato's character, 'Socrates'. If we are to believe that Plato himself wrote the Seventh Letter, it must follow that he is implying here that the Socrates of the Republic is him, or at any rate his spokesman. Is that credible?Schofield then argues that it is hardly credible that (i) Plato "would speak as though the Republic were not a dialogue, but a pronouncement in his own person", or that, (ii) although recognizing that it was a dialogue, Plato nonetheless "saw Socrates as his spokesman" in the dialogue.
Against (i): Plato "might have expounded views in his own person in a continuous discourse" but instead "he invests huge energy and remarkable literary ingenuity in the creation of philosophical dramas--and dramatis personae to people them--in which he must in some sense be all and none of the characters whose voices are heard in the conversation." Schofield quotes John Cooper approvingly: "It is in the entire writing that that author speaks to us, not in the remarks made by individual speakers."
Against (ii): Schofield favors the view put forward by David Sedley (in his Cratylus book), who "has called attention to the analysis in several later dialogues of thinking itself as an internal dialogue of question and answer. [Sedley] suggests that the question-and-answer sequences in the dialogues constitute externalizations of Plato's own thought processes. That is how Plato maintains a 'dominating and inescapable presence' in the dialogues. An answer to the question of authorial point of view suggests itself. The dialogues are to be read as 'Plato thinking aloud'."
(Schofield distingushes (i) and (ii), but I don't see how he succeeds in keeping these distinct. His answers look like answers to the same question. "Does a dialogue say something; does it make a statement or pronounce something?" That's not answered by pointing out that the author took pains to write in dialogue form. Yet once one adds: "the author does not speak to us in the remarks made by individual speakers", one has moved on to answering the other question.)