First, because, since everything desires its own perfection, a man desires for his ultimate end, that which he desires as his perfect and crowning good. Hence Augustine (De Civ. Dei xix, 1): "In speaking of the end of good we mean now, not that it passes away so as to be no more, but that it is perfected so as to be complete." It is therefore necessary for the last end so to fill man's appetite, that nothing is left besides it for man to desire. Which is not possible, if something else be required for his perfection. Consequently it is not possible for the appetite so to tend to two things, as though each were its perfect good.