As literature, the dialogues of Plato's middle period are among the world's greatest creative achievements; the later dialogues fall short of them in imaginative power and dramatic skill. Nevertheless they advance considerations of great importance for the future of logic and metaphysics. Plato was now much concerned with the theory of knowledge. A fairly early dialogue, the Meno, had pointed the way; Meno, an intelligent but uneducated slave, is questioned by Socrates and shown to discover a simple mathematical truth without being told. Some truths, then, can be known independently of experience; and Plato concludes that the soul became acquainted with the Forms in a previous existence which we have forgotten; the discovery of such truths is in fact a recollection (anamnēsis). This clearly marks a distinction between knowledge of the Forms and knowledge of everyday facts; but the proof of our pre-existence gives little support to the theory of transmigration, which Plato presents in several dialogues with a wealth of imaginative detail; for we are said to recollect a previous ideal existence, whereas the transmigration theory would make it probable that other imperfect incarnations have preceded our present life.

In the Theaetetus, where the problem of knowledge is more fully discussed, there is surprisingly little reference to the Forms; but the dialogue is important, inter alia, for its demonstration that perceptual knowledge involves more than mere perception, and again for the suggestion that knowledge is a disposition; knowing is not something like seeing or sleeping which we do from time to time; to know something is to be able to act or answer correctly when required.