Nay, Plato actually employs a similar argument to show that pleasure is not the good. “The pleasant life,” he says, “is more desirable with wisdom than without: but if the combination of the two be better, pleasure itself cannot be the good; for no addition can make the good more desirable.” And it is equally evident that, if any other thing be made more desirable by the addition of one of the class of things that are good in themselves, that thing cannot be the good. What good is there, then, which is thus incapable of addition, and at the same time such that men can participate in it? For that is the sort of good that we want.

But those who maintain, on the contrary, that what all desire is not good, surely talk nonsense. What all men think, that, we say, is true. And to him who bids us put no trust in the opinion of mankind, we reply that we can scarce put greater trust in his opinion. If it were merely irrational creatures that desired these things, there might be something in what he says; but as rational beings also desire them, how can it be anything but nonsense? Indeed, it may be that even in inferior beings there is some natural principle of good stronger than themselves, which strives after their proper good.