Plato, answered from the texts and cited to the page.
The Form of the Good is the hardest thing I have tried to say, and I will not pretend otherwise. What I can offer is the analogy, because the thing itself resists direct statement. Consider how sight works. The eyes may be present in a face, colors may be present in objects, and yet if there is no light, nothing is seen. Light is the third thing, the link between the power of seeing and the power of being seen.1
And the cause of that light — the god in heaven whose gift it is — is the sun. Now, sight is not the sun; the eye is not the sun; but sight is the most sunlike of the senses, receiving from the sun the power it has, as if from an overflowing treasury.2 The sun is not sight, yet it is the cause of sight itself, and is seen by it.3 So it stands with the Good in the intelligible realm.
When the soul fixes its attention on what is illuminated by truth and what genuinely is, it understands and knows; when it turns toward what is mixed with obscurity, toward what comes to be and passes away, it merely opines, shifts its views, and seems bereft of understanding.4 What gives truth to the things known, and the power to know to the knower, is the Form of the Good.
It is the cause of knowledge and truth, and is itself an object of knowledge — yet it is other and more beautiful than either.5 As the sun gives visible things not only the power to be seen but their very coming-to-be, growth, and nourishment, so the Good in the intelligible realm controls and provides truth and understanding, and is the cause of all that is correct and beautiful in anything.6
This is why I say it is the last thing to be seen in the knowable realm, and reached only with difficulty. The philosopher who ascends to it — who has turned the eye of the soul from the shadows of the cave upward through mathematics and dialectic to the Form itself — must afterward conclude that it is the source of everything. Anyone who is to act sensibly in private or public must see it.7
Those who do reach it are, not surprisingly, reluctant to descend again into human affairs; their souls press upward, and when they are compelled to return and dispute about the shadows of justice with people who have never seen justice itself, they appear awkward and ridiculous.8 There is a further implication, one I draw out in the dialogue with Alcibiades.
The soul that would know anything must first know itself — must look at that region in itself where knowing and understanding take place, the region most divine.9 The Form of the Good is what that region is oriented toward when it functions at its best. To know oneself and to know the Good are not two separate tasks; self-knowledge and knowledge of what is highest are aspects of a single ascent.
I said to Glaucon that I was certainly omitting a great deal, and I meant it.10 The Good is not a proposition to be extracted from the dialogue and carried away. It is what the dialectical ascent is toward, and the ascent is what the dialogues enact. Whether the account is fully true, only the god knows — but this is how I see it.
Sight may be present in the eyes, and the one who has it may try to use it, and colors may be present in things, but unless a third kind of thing is present, which is naturally adapted for this very purpose, you know that sight will see nothing, and the colors will remain unseen.Complete Works, pp. 1157–1158
The sun is not sight, but isn't it the cause of sight itself and seen by it?... it receives from the sun the power it has, just like an influx from an overflowing treasury.Complete Works, pp. 1157–1158
Let's say, then, that this is what I called the offspring of the good, which the good begot as its analogue. What the good itself is in the intelligible realm, in relation to understanding and intelligible things, the sun is in the visible realm, in relation to sight and visible things.Complete Works, pp. 1157–1158
When it focuses on something illuminated by truth and what is, it understands, knows, and apparently possesses understanding, but when it focuses on what is mixed with obscurity, on what comes to be and passes away, it opines and is dimmed, changes its opinions this way and that, and seems bereft of understanding.Complete Works, pp. 1157–1158
So that what gives truth to the things known and the power to know to the knower is the form of the good. And though it is the cause of knowledge and truth, it is also an object of knowledge. Both knowledge and truth are beautiful things, but the good is other and more beautiful than they.Complete Works, pp. 1158–1160
in the intelligible realm it controls and provides truth and understanding, so that anyone who is to act sensibly in private or public must see it.Complete Works, p. 1164
the form of the good is the last thing to be seen, and it is reached only with difficulty. Once one has seen it, however, one must conclude that it is the cause of all that is correct and beautiful in anything.Complete Works, p. 1164
What about what happens when someone turns from divine study to the evils of human life? Do you think it's surprising, since his sight is still dim, and he hasn't yet become accustomed to the darkness around him, that he behaves awkwardly and appears completely ridiculous if he's compelled, either in the courts or elsewhere, to contend about the shadows of justice.Complete Works, p. 1164
if the soul, Alcibiades, is to know itself, it must look at a soul, and especially at that region in which what makes a soul good, wisdom, occurs... Can we say that there is anything about the soul which is more divine than that where knowing and understanding take place?Complete Works, p. 621
I'm certainly omitting a lot... I have to omit a fair bit, but, as far as is possible at the moment, I won't omit anything voluntarily.Complete Works, pp. 1158–1160