Menexenus

... It seems impossible to separate by any exact line the genuine writings of Plato from the spurious. The only external evidence to them which is of much value is that of Aristotle; for the Alexandrian catalogues of a century later include manifest forgeries. Even the value of the Aristotelian authority is a good deal impaired by the uncertainty concerning the date and authorship of the writings which are ascribed to him. And several of the citations of Aristotle omit the name of Plato, and some of them omit the name of the dialogue from which they are taken...