Plato
Light

First we must come to an understanding. Let me remind you of the distinction we drew earlier and have often drawn on other occasions, between the multiplicity of things that we call good or beautiful or whatever it may be and, on the other hand, Goodness itself or Beauty itself and so on. Corresponding to each of these sets of many things, we  postulate a single Form or real essence , as we call it.

Yes, that is so.

Further, the many things, we say, can be seen, but are not objects of rational thought; whereas the Forms are objects of thought, but invisible.

Yes, certainly.

And we see things with our eyesight, just as we hear sounds with our ears and, to speak generally,  perceive any sensible thing with our sense-faculties.

Of course.

Have you noticed, then, that the artificer who designed the senses has been exceptionally lavish of his materials in making the eyes able to see and their objects visible?

That never occurred to me.

Well, look at it in this way. Hearing and sound do not stand in need of any third thing, without which the ear will not hear nor sound be heard; and I think the same is true of most, not to say all, of the other senses. Can you think of one that does require anything of the sort?

No, I cannot.

But there is this need in the case of sight and its objects. You may have the  power of vision in your eyes and try to use it, and color may be there in the objects; but sight will sec nothing and the colors will remain invisible in the absence of a third thing  peculiarly constituted to serve this very  purpose.

By which you mean?

Naturally I mean what you call light; and if light is a thing of value, the sense of sight and the power of being visible are linked together by a very  precious bond, such as unites no other sense with its object.

No one could say that light is not a  precious thing.

And of all the divinities in the skies is there one whose light, above all the rest, is responsible for making our eyes see  perfectly and making objects  perfectly visible?

There can be no two opinions: of course you mean the Sun.

And how is sight related to this deity? Neither sight nor the eye which contains it is the Sun, but of all the sense-organs it is the most sun-like; and further, the power it  possesses is dispensed by the Sun, like a stream flooding the eye. And again, the Sun is not vision, but it is the cause of vision and also is seen by the vision it causes.

Yes.

It was the Sun, then, that I meant when I spoke of that offspring which the Good has created in the visible world, to stand there in the same relation to vision and visible things as that which the Good itself bears in the intelligible world to intelligence and to intelligible objects.

How is that? You must explain further.

You know what happens when the colors of things are no longer irradiated by the daylight, but only by the fainter luminaries of the night: when you look at them, the eyes are dim and seem almost blind, as if there were no unclouded vision in them.

But when you look at things on which the Sun is shining, the same eyes see distinctly and it becomes evident that they do contain the power of vision.

Certainly.

Apply this comparison, then, to the soul. When its gaze is fixed upon an object irradiated by truth and reality, the soul gains understanding and knowledge and is manifestly in  possession of intelligence. But when it looks towards that twilight world of things that come into existence and  ass away, its sight is dim and it has only opinions and beliefs which shift to and fro, and now it seems like a thing that has no intelligence.

That is true.

 This, then, which gives to the objects of knowledge their truth and to him who knows them his power of knowing, is the Form or essential nature of Goodness. It is the cause of knowledge and truth; and so, while you may think of it as an object of knowledge, you will do well to regard it as something beyond truth and knowledge and, precious as these both are, of still higher worth. And, just as in our analogy light and vision were to be thought of as like the Sun, but not identical with it, so here both knowledge and truth are to be regarded as like the Good, but to identify either with the Good is wrong. The Good must hold a yet higher place of honor.

You are giving it a position of extraordinary splendor, if it is the source of knowledge and truth and itself surpasses them in worth. You surely cannot mean that it is pleasure.

Heaven forbid, I exclaimed. But I want to follow up our analogy still further. You will agree that the Sun not only makes the things we see visible but also brings them into existence and gives them growth and nourishment; yet he is not the same thing as existence. And so with the objects of knowledge: these derive from the Good not only their power of being known, but their very being and reality; and Goodness is not the same thing as being, but even beyond being, surpassing it in dignity and power.

Glaucon exclaimed with some amusement at my exalting Goodness in such extravagant terms.

It is your fault, I replied; you forced me to say what I think.

Yes, and you must not stop there At any rate complete your comparison with the Sun, if there is any more to be said.

There is a great deal more, I answered.