Image of the “Cave”
Plato’s illustration in the Republic Book VII (ΠΟΛΙΤΕΙΑ) of the difference between knowledge and illusion, reality and appearance. Men chained in a cave, facing a blank wall, with a fire burning behind them, can see only shadows, which they take for real objects.
When one who has been made to leave the cave and see the real world by the light of the sun returns, it is hard for him to adapt to the dim light; he is ridiculed by his former companions and is unable to convince them that what they see are but vague reflections of reality.
The Theory of Forms
The insistence that the objects of knowledge must be Forms is the result of the requirement that what is known must be true, helped along by a Greek idiom that suggests that characteristics are substances (ΟΥΣΙΑΙ).
If what is known must be true without qualification (the exaggeration) then it must be possible to say it is without qualification,but one can only say this is Form. No object of sensory experience can be described as just, beautiful, large, or heavy without qualification. Such things will always be unjust, ugly, small, or light in comparison with something else. Hence there can be no knowledge of the justice, beauty, size, or weight of sensory objects- at best only true belief.
The Theory of Reminiscence (ΑΝΑΜΝΗΣΙΣ)
How it might be possible actually to arrive at a definition is answered in one way by the doctrine of reminiscence or recollection (anamnesis). This first appears in an early dialogue, the Meno (ΜΕΝΩΝ,) where Plato suggests that we are born already in possession of knowledge of which we are not conscious but which we will readily recollect if carefully prompted.
He has Socrates illustrate this by drawing the answer to a geometrical problem out of a slave boy who knows no geometry.
A special Form, the Good (ΤΟ ΑΓΑΘΟΝ)
No connection is made in the Meno (ΜΕΝΩΝ) between recollection and Form; this is done in the Phaedo (ΦΑΙΔΩΝ).
However, no mention of recollection is made in the Republic’s (ΠΟΛΙΤΕΙΑ) extensive discussion of a discipline called “dialectic” (ΔΙΑΛΕΚΤΙΚΗ )which is supposed to study the forms. This discipline evidently has affinities to Socrates’ practice in the early dialogues, since it is forbidden to those under 30 because youths too readily indulge in refutation for its own sake and are vulnerable to disillusionment from witnessing too many refutations.
The difference seems to be that Plato thought he had hit on a systematic application of the early practice of dialectic. The system haw something to do with laying down and testing hypotheses, but Plato goes on to picture the advance of dialectic up a ladder of hypotheses – at the top of which is a special Form, the Good – without leaving us any clear account of how to get from one rung of the ladder to the next.