Because when we wish to prove some particular good, we either prove by defining that the same description applies to the good and to the thing which we wish to prove to be good, or else have recourse to induction; for instance, when we wish to prove that magnanimity is a good, we say that justice is a good and courage is a good, and so of the excellences generally, and that magnanimity is an excellence, so that magnanimity also is a good.