Russell says in his History of Western Philosophy that Burnet suggests that Xenophon is copying Plato in depicting Socrates. This served Russell as a major factor in his refraining from discussing the historical Socrates, and, in this respect, the majority of classical philosophers are in harmony with Bertrand Russell. So let us examine the reasons that led Burnet to his view.
John Burnet says in the ‘Introduction’ to his edition of Plato’s Phaedo: ‘If only we may take the Phaedo for what it professes to be, it surely stands quite by itself in European literature. It does not, indeed, claim to be a word for word report of all Socrates said to the inner circle of his followers on the day he drank the poison in prison. By letting us know incidentally (59b10) that he was not present, Plato seems to decline responsibility for the literal exactitude of every detail … We are certainly led to believe that it gives us a truthful record of the subjects on which Socrates discoursed on the last day of his life, and of his manner of treating them. No reader who made his first acquaintance with Socrates here could possibly suppose anything else … it is the likeness of a great philosopher in the supreme crisis of his life, drawn by a philosopher who was greater still, and was also one of the most consummate dramatic artists the world has known. It would not be easy to find the match of such a work.’ (Clarendon Press, Oxford, 1911, reprinted 1977, pp. IX-X)
In his effort to give back to the reader the possibility of re-living the last day of Socrates with his disciples Burnet found it imperative to discredit Xenophon as an independent source of information about Socrates: ‘The interpretation which finds nothing in the Phaedo but the speculations of Plato himself is based on the belief that “the historical Socrates”, of whom we may get some idea from Xenophon, is quite a different person from “the Platonic Socrates”. What the latter is made to say is treated as evidence for the philosophy of Plato, but not for that of Socrates himself … The general acceptance of this in recent times is apparently due to the authority of Hegel. Speaking of Socrates, he lays down that “we must hold chiefly to Xenophon in regard to the content of his knowledge, and the degree in which his thought was developed” [Gesch. Der Phil. II. 69), and this dictum became a sort of dogma with the Hegelian and semi-Hegelian writers to whom we owe so much of the best nineteenth-century work in the history of Greek philosophy.’ (pp. XII-XIII)
Burnet protested against this view of the Phaedo: ‘I cannot bring myself to believe that Plato falsified the story of his master’s last hours on earth by using him as a mere mouthpiece for novel doctrines of his own.’ (p. XI)
Burnet’s attempt to give the Phaedo back to the reader failed miserably. A whole section of the Sixth International Congress of Philosophy, held in 1927 in New York, was devoted to the condemnation of his views. (See The Lost Plato on my website, ‘The Introduction’) What nevertheless prevailed was his discrediting of Xenophon as a source of information on the historical Socrates.