Frede, Dorothea, "The Impossibility of Perfection: Socrates' Criticism of Simonides' Poem in the Protagoras," The Review of Metaphysics 39, no. 4 (June 1986), 729-53.

Excerpt:

The claim that even Plato could not say everything at once nor could have thought or worked out everything at once is, of course, a platitude. It is generally acknowledged that there is development in Plato’s thought. But what the development is, is still a much fought-over question. For in spite of all scholarly efforts this intriguing question cannot be regarded as settled in a satisfactory way.

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