For you have often heard that the greatest thing to learn is the idea of good by reference to which just things and all the rest become useful and beneficial. And now I am almost sure you know that this is what I am going to speak of and to say further that we have no adequate knowledge of it. And if we do not know it, then, even if without the knowledge of this we should know all other things never so well, you are aware that it would avail us nothing, just as no possession either is of any avail without the possession of the good.
Now it is true that this argument has a certain plausibility; but it does not seem to square with the actual procedure of the sciences. For these all aim at some good, and seek to make up their deficiencies, but they do not trouble about a knowledge of the Ideal Good. Yet if it were so potent an aid, it is improbable that all the professors of the arts and sciences should not know it, nor even seek to discover it. Moreover, it is not easy to see how knowing that same Ideal Good will help a weaver or carpenter in the practice of his own craft, or how anybody will be a better physician or general for having contemplated the absolute Idea.
In the Republic, Socrates makes knowledge of the Good the condition of every other benefit: knowing all else without it would be worth nothing. Aristotle grants the thought some plausibility, then points out that the arts and sciences pursue their goods without any such knowledge, so a separate Good would be of no use for action even if it existed. Earlier in the chapter he argues that "good" is said in as many ways as "is" and cannot fall under one Form. He names the theory and its authors, his friends, but not the dialogue; Republic VI is the fullest statement of it that survives.
