Cropsey, Joseph, "Apology of Socrates," Plato's World: Man's Place in the Cosmos, Chicago, IL: University of Chicago Press, 1995.

Excerpt:

There are at least three principles on which the Dialogues of Plato, or some of them, can be arranged to form a general schema. The first to be employed was the ancient grouping of the dialogues in the famous tetralogies according to their perceived subject or other attribute. This principle, no longer popular, produced a result described in detail by Diogenes Laertius (3.57–62). In later times the effort has been made repeatedly and of necessity inconclusively to arrange the dialogues in order of their composition, perhaps on the basis of perceived differences in the writing style presume to be characteristic of a younger or older author, such speculation fortified when possible with the more objective information provided by occasional internal references to historic events.

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