Plato raises the idea that the (human) good might be pleasure or knowledge, but dismisses both. It can’t be pleasure, he says, for then any man who was pleased would be a good man; and it can’t be knowledge since knowledge is of something, and so all such an answer would amount to is answering the question “what is the good?” with “knowledge of the human good”, to which the obvious question is “so…what is the good?” These two answers might provide a general template for problems defining the human good: either we give something that could be present in someone not good, or we presuppose knowledge of the very good we’re trying to find.
Plato answers the question by analogy. Just as the eye has a double relation, on the one hand to a visible object and in another way to the sun, so too the soul has a double relation, both to the various similitudes of things in the world and to the good. By soul we should assume he means any sort of human awareness, not just for intellectual things but for moral things as well (Plato seemed less interested than Aristotle in the way in which these things are different). And so the good is what stands to the soul as the sun stands to the eye: that to which we have a natural ordination and in virtue of which anything given in awareness can constitute a world belonging to the soul, that is, a moral, intellectual, poetic or social world.
But what is the good then? Plato seems to be shifting the question, or at lest giving a very startling response. The good is not something in the world, whether riches or pleasure or knowledge or even virtue. The good properly seems to be that which constitutes the world of awareness as a world at all. The fundamental division in the human soul is on the one had towards any good that might be in the world, or even the world itself as a sort of good, and on the other hand to a transcendent source which can only be understood in opposition to a good given in awareness. To treat the transcendent good as though it was just another good like pleasure, knowledge, or even virtue is to miss the very way we come to know it. The transcendent good is no thing – it is even helpful to see it as nothing or at lest as athing (“not a thing but athing” is a catchy way to put it.) But this line of sheer negation can’t be absolute.
There is a temptation to see this athing as God, but this is too quick. The knowledge of athing is useful to theology but it is not limited to that. There is more than one thing that is athing. Even things themselves are somehow athing insofar as they have that within themselves by which they constitute a world. If there are logical problems with saying this, so much for logic in this particular application.