Averroes' Paraphrase of Plato's Republic

Therefore, since it is clear from the above what the purpose of a just war is, it will not seem irrelevant now to transfer the discourse to each individual point which has been argued by Plato concerning the selection of those apt for such virtues, and by what sagacity the virtues themselves are confirmed in their minds, and that they themselves exercise them in such a way that the actions proceeding from those virtues are the best. We believe that Plato himself judged that no one in the city should be instructed in more than one art. Indeed, since no one is naturally suited for multiple arts. To these, any person better attains the habit of the art itself, so that he might produce a perfect work of art, if he has been educated in it from boyhood. Just as no one in equestrian games and those assemblies of armed knights colliding with hostile spears achieves what he desires, unless he has been assiduously and plainly instructed in both matters from boyhood, how much less will he achieve it in the military art? Add also that several arts share both reason and time among themselves, from which it is necessary that the works of those who have attacked multiple arts be disturbed and entangled. And for this reason, Plato opined that the guardians should be experts in the other arts.
that no one should be instructed in more than one art