As we know little about Plato except from his works, the first question to be decided is, Which are his real works? Where are we to find a trustworthy Platonic Canon?

Platonic Canon—Ancient and modern discussions

Down to the close of the last century this question was not much raised or discussed. The catalogue recognised by the rhetor Thrasyllus (contemporary with the Emperor Tiberius) was generally accepted as including none but genuine works of Plato; and was followed as such by editors and critics, who were indeed not very numerous. But the discussions carried on during the present century have taken a different turn. While editors, critics, and translators have been greatly multiplied, some of the most distinguished among them, Schleiermacher at the head, have either professedly set aside, or in practice disregarded, the Thrasyllean catalogue, as if it carried no authority and very faint presumption. They have reasoned upon each dialogue as if its title to be considered genuine were now to be proved for the first time; either by external testimony (mentioned in Aristotle or others), or by internal evidences of style, handling, and thoughts: as if, in other words, the onus probandi lay upon any one who believed the printed works of Plato to be genuine—not upon an opponent who disputes the authenticity of any one or more among them, and rejects it as spurious. Before I proceed to examine the conclusions, alike numerous and discordant, which these critics have proclaimed, I shall enquire how far the method which they have pursued is warrantable.