Plato’s Republic Vol VI (509b)

Just in the same way understand the condition of the soul to be as follows. Whenever it has fastened upon an object, over which truth and real existence are shining, it seizes that object by an act of reason, and knows it, and thus proves itself to be possessed of reason: but whenever it has fixed upon objects that are blent with darkness,— the world of birth and death,—then it rests in opinion, and its sight grows dim, as its opinions shift backwards and forwards, and it has the appearance of being destitute of reason.

True, it has.

Now, this power, which supplies the objects of real knowledge with the truth that is in them, and which renders to him who knows them the faculty of knowing them, you must consider to be the essential Form of Good, and you must regard it as the origin of science, and of truth, so far as the latter comes within the range of knowledge: and though knowledge and truth are both very beautiful things, you will be right in looking upon good as something distinct from them, and even more beautiful. And just as, in the analogous case, it is right to regard light and vision as resembling the sun, but wrong to identify them with the sun; so, in the case of science and truth, it is right to regard both of them as resembling good, but wrong to identify either of them with good; because, on the contrary, the quality of the good ought to have a still higher value set upon it.

That implies an inexpressible beauty if it not only is the source of science and truth, but also surpasses them in beauty; for, I presume, you do not mean by it pleasure.

Hush! I exclaimed, not a word of that. But you had better examine the illustration further, as follows.

Shew me how.

I think you will admit that the sun ministers to visible objects, not only the faculty of being seen, but also their vitality, growth, and nutriment, though it is not itself equivalent to vitality.

Of course it is not.

Then admit that, in like manner, the objects of knowledge not only derive from the good the gift of being known, but are further endowed by it with a real and essential existence; though the good, far from being identical with real existence, actually transcends it in dignity and power.

Hereupon Glaucon exclaimed with a very amusing air, Good heavens! what a miraculous superiority!

Well, I said, you are the person to blame, because you compel me to state my opinions on the subject.

Nay, let me entreat you not to stop, till you have at all events gone over again your similitude of the sun, if you are leaving anything out.

Well, to say the truth, I am leaving out a great deal.

Then pray do not omit even a trifle.

I fancy I shall leave much unsaid; however, if I can help it under the circumstances, I will not intentionally make any omission.

Pray do not.

Now understand that, according to us, there are two powers reigning, one over an intellectual, and the other over a visible region and class of objects;—if I were to use the term “firmament” * you might think I was playing on the word. Well then, are you in possession of these as two kinds,—one visible, the other intellectual?