What is the relationship between knowledge and light that Plato presents in the Cave Allegory in Book Seven of his Republic?
Light - 1
In the cave allegory Plato repeats the relationship between knowledge and light that he had posited in the simile of the sun (Rep., VI, 507a ff.). The sun is the Form of the Good, the ultimate principle of reality and goodness, and the light experienced on release from the cave is truth. The Form of the Good produces truth just as the sun produces light. What it produces truth about is the objects of knowledge, namely the Forms. Our faculty of reason (nous) acquaints us with these objects; it renders them intelligible or knowable just as our faculty of sight enables us to see the objects of sense or perception. Or rather, not quite ‘just as’ because although we can have knowledge of the Forms we cannot have knowledge, strictly speaking, of the objects or sense or perception because these roll between being and non-being (Rep., V, 477a6-478e5). They are not fully real, though we’d have to delve into the thickets of Plato’s metaphysics to see in what sense and exactly why. But a quick answer is that the objects of sense or perception have different properties across time and stand in different mutual relations; they also come into existence and pass away. For Plato ultimate reality is not unstable in this way but absolutely unchanging.
Light - 2
What, though, of the ‘light burning from a distant fire’ behind the prisoners ? (Rep., VII. 514b)? This is certainly not sunlight, yet it is light. The light of the fire produces only shadows, which are all the prisoners are visually acquainted with. The prisoners also hear sounds but all they see are shadows.
So far as I can make out, the prisoners represent the ordinary citizens of the polis, those who have not had the guardians’ education. The prisoners see only the shadows of objects, not the objects themselves. Likewise the ordinary citizens have mere opinion (doxa) when they experience the objects of sense or perception, since knowledge of this unstable world is impossible. They see only ‘shadows’ of the Forms insofar as the objects of sense or perception ‘participate’ or ‘imitate’ them (methexis, mimesis). For participation see Rep., V.476d; for imitation, Rep. X, 597a ff.. The fire-light of the cave is a kind of imperfect imitation of sunlight, the light outside the cave. In Plato’s metaphorical language, fire-light produces not truth and genuine knowledge of ultimate reality but only opinion about the unstable world of the senses.
Plato, Republic, bks V – VII. Ed. G.R.F. Ferrari, tr. T. Griffith, Cambridge: CUP, 2015.
James Adam, ed. The Republic of Plato, Cambridge: CUP, 1902, II: 60.