“I understand that less than I understand the Good of Plato,” says a slave in comedy. The slave's understanding of the Good would not have been much helped by attending Plato's own public lecture on the subject: the majority of the audience “came in the expectation of acquiring some of those things that conventionally count as human goods, such as health, wealth, strength, and in general some wonderful happiness; but when the discussion turned out to be on the mathematical sciences - numbers, geometry, astronomy - and the conclusion to be that good is one, this struck them as utterly paradoxical; whereupon some came to despise the business, and others started to make complaints.” Nevertheless, in one respect at least, it is actually quite easy to grasp what Plato has to say about the Good.
If for the moment we confine our attention to Forms of artifacts, it is easy to understand and accept the Republic's claim that the Good has the privileged position of being what accounts for the existence and intelligibility of Forms, much as the Sun has the privileged position of being what accounts for the growth and visibility of plants (508b-e, 509b). For the claim will then be that everything about an ideal artifact is teleologically explicable. The ideal wheel is circular, and it has its axle in its center. These things are so because that is the best way for a wheel to be: a buckled wheel, or for that matter a circular wheel with its axle off-center, would give a bumpy ride. And the Good accounts in this way for every aspect of the ideal wheel: if, for example, we ask about the color of the ideal wheel, then the only answer is that it has no particular color; and that is because there is no one color that is the best one for a wheel to have.