Quotation by Anselm of Canterbury

Therefore even the fool agrees that that than which a greater cannot be thought exists in the understanding, because he understands this when he hears it and whatever is understood is in the understanding. And certainly that than which a greater cannot be thought cannot exist only in the understanding. For if it exists only in the understanding, then it can be thought to exist in reality, which is greater. Therefore, if that than which a greater cannot be thought is in the understanding only, then that than which a greater cannot be thought can be thought to be greater. But certainly this is impossible.