The Fifth Letterpurports to be a private letter addressed to Perdiccas III, king ofMacedon from 365 to 359 BCE. It announces that its author has complied with Perdiccas' request to counsel one Euphraeus to turn his attention to Perdiccas' interests and then proceeds to counsel Perdiccas himself on the advantages of listening to Euphraeus' counsels. Perdiccas is young, and few people are as fit to give advice concerning politics as they claim to be. Echoing the Republic, the letter goes on to say that each form of government has its own language or voice, and that it is necessary to speak to both gods and men in the voice appropriate to the regime if one is to flourish. Euphraeus will help Perdiccas to explore the speech or logic of monarchy. The letter next raises a hypothetical objection: Plato himself did not speak to the Athenian democracy, even as the above argument suggests that he knows what would be advantageous to it. It counsels Perdiccas to respond to this objection by saying that Plato was born when his fatherland had already been corrupted beyond the ability of his counsel to benefit it, while the risks of engaging in its politics were great, and that he would probably leave off counseling Perdiccas himself if he thought him incurable.
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Authenticity
The Fifth Letter is one of the four epistles that has very little support as to its authenticity. Both R. G. Bury and L. A. Post consider it to be spurious, though Hamilton and Cairns suggest that it does have at least some defenders, without defending it themselves. The arguments against its authenticity rely mainly on the unsatisfactory character of its content. Bury notes that the discussion of the "voices" of various regimes is borrowed from the Republic and that the explanation of when it is beneficial to give counsel seems borrowed from the Seventh Letter; this suggests that the author had these works before his eyes and was conscientiously trying to sound like Plato when he was writing the Fifth Letter, which would more likely be the case if he were a forger than Plato attempting to be consistent with himself. He also notes that there is no need to defend Plato's abstention from politics given the ostensive purpose of the letter, which is to recommend Euphraeus' competence. Having concluded from this that Plato is not the author, Bury speculates that, "Unless the writer were himself a monarchist, the ascription of this letter to Plato may have been due to a malicious desire to paint Plato as a supporter of Macedon and its tyrants." There is some suggestion that Plato did have some relationship with Perdiccas, though it is difficult to determine the degree to which the Fifth Letter influenced this perception, and thus the relevance of this material in examining its authenticity. A fragment of Carystius' Historical Notes preserved by Athenaeus in Book XI of the Deipnosophistae reports that "Speusippus, learning that Philip was uttering slanders about Plato, wrote in a letter something of this sort: 'As if the whole world did not know that Philip acquired the beginning of his kingship through Plato’s agency. For Plato sent to Perdiccas Euphraeus of Oreus, who persuaded Perdiccas to portion off some territory to Philip. Here Philip kept a force, and when Perdiccas died, since he had this force in readiness, he at once plunged into the control of affairs.'" Demosthenes notes in his Third Philippic that Euphraeus once resided in Athens, and portrays him as being active in politics, albeit in opposition to Philip.
