Plato’s simile of light – the images of the Sun, the Divided Line and the Cave are outlined in the Republic at the close of Book VI and at the beginning of Book VII. The simile of light has attracted a vast literature from Nettleship’s Victorian lectures, down through the work of James Adam, Henry Jackson and A.S. Ferguson’s brilliant series of articles, to the more recent work of Cross and Woozley, Raven, White and Annas. Of course, there are too many shades and possibilities of interpretation to be canvassed fully in such a limited space but I have been influenced at different points by different scholars. I merely put forward the views I hold about the context, functionality and continuity of the three images.
The simile of light has both a “forward” and a “backward” context. By the forward context I mean that the simile anticipates, and is a metaphorical representation of, the education of the guardians as spelt out by Plato in the remainder of Book VII, after the Cave. And it is through this education that the philosopher will come to know the Form of the Good, the foundation of all value, to be able to govern effectively. So when Socrates presses men to define, what is meant by justice? they resort to characterizing justice in terms of certain just acts. To avoid ethical particularism Socrates needs to abstract from the individual action. So if you claim to know what justice is then presumably you can capture the conditions for justice in a definitional claim. Socrates thus appeals to a universalizability, i.e. the Form of Justice or any other virtue which ultimately participates in the one great universal, the Good. Just as the Sun provides a teleological picture of the world (Rees 1965: xxxv) so too by analogy, consciously or unconsciously, does man participate in the Good. This is reiterated lucidly by Boyd (1922: 128) when he says that ‘in effect that what justice or any other virtue is, we must see it in its relations to life as a whole’.
Now to the “backward” context. Plato does say what the Form of the Good is not – it is not pleasure and it is not knowledge. The point is: can he reasonably assume that there is such a thing, given the metaphysical arguments of the Republic so far? It’s the metaphysical role of the Form of the Good that’s primarily important to Plato. He feels the need for an ultimate explanatory principle – and the Form of the Good fits the bill. There is the temptation to view the Form of the Good as omni-explanatory (Cross and Woozley 1964: 183 cite Cherniss as commending the theory for its philosophical economy), a framework for a kind of theodicy or impersonal God. The Form of the Good “only explains the existence of goodness wherever goodness occurs”. The Form of the Good is the principle of reality, since goodness and reality are interrelated, and is fundamental to any attempt to making the world intelligible. Plato is thus committed to the idea that there is no difference of ultimate nature between facts and values in the world: goodness and values are just as real or indeed more real than other things. Just as the sun provides light, the intermediary between the eye and its object, so the Good provides the intermediary between the mind and its object, thereby making knowledge possible. The Line further illustrates this relation between the two orders of reality, the visible and the intelligible, but from a cognitive point of view – the states of mind in which one apprehends.
That the sun is primarily metaphysical in purpose is the least contentious of the three similes’ interpretations. The Sun is the child of the Good (508b) occupying in the visible world a position analogous to that of the Form of the Good in the world of Forms. Without the Good other Forms would not be known. Further, all the universals whether they be moral properties or mathematical entities (perfect virtue, perfect circles etc.) necessarily exist even independently of any particulars to exemplify them. The Good by analogy with the Sun is the source not only of their intelligibility, “but also of their being and reality; yet it is not itself that reality, but is beyond it, and superior to it in dignity and power” (509b). Likewise “the Sun . . . not only makes the things we see visible, but causes the processes of generation, growth and nourishment, without itself being such a process.”
I do need to reiterate that Plato does not suggest that the Idea of the Good provides a complete explanation of the entire universe, comprising both the intelligible and sensible realms, recalling the comments on the omni-explanatory function of the Form of the Good in the previous section. Cross and Woozley (1964: 183) make this point that the “completely real” world of Forms cannot deny the “semi-real” world of our banal reality. It is a common inference that because Plato constructs only one line, i.e. a common scale, that its segments represent a progression through its four stages. This orthodoxy (Nettleship 1901: 238-258) emphasizes this progression or “passing” that the mind needs to go through. Ferguson (1921: 147-150) refutes the view arising from the assumption of the Line’s being an exhaustive classification by maintaining that the Line is a continuation of the Sun simile (indeed as Plato says at 509c) whereby the Line simile is an illustration of “how two successive methods of studying the intelligible may lead to knowledge of that transcendent Good, still using the convenient symbolism of the visible” (Ferguson 1921:136), highlighting the contrast between the two methods of mathematics and philosophy, represented in the two upper segments.