Averroes' Paraphrase of Plato's Republic

For this reason, among other things, Plato was led to believe that no one ought to devote effort to only one art, and this will be explained by us a little later. Therefore, for someone to practice several arts is either altogether impossible, or if it should be possible, it will be done not at all excellently.

Human perfections can only be perceived when separated, yet within a certain definite multitude.

And since human perfections can be perceived only when separated, yet all within a certain definite multitude, therefore individual men differ among themselves in their birth and creation according to the diversity of the said perfections.
Individuals cannot possess all perfections.

For if individuals could attain all human perfections, nature would have acted rashly and in vain. Since it is impossible that any possible thing be given that cannot be reduced to act. And that has already been explained in natural Philosophy. It is also confirmed by sense that individual men are thus. This is indeed most evidently plain in the more excellent virtues: because not everyone is by nature accommodated to turn out to be either warlike, or an orator, or a poet, and especially a philosopher. If the matter stands (as we have said) in this way, it is consonant with reason that all human perfections be found in some assembly of men; and that they should in turn assist one another toward perfection, so that he who possesses some virtue imperfectly may flee to him who has attained this same virtue most exactly.