“It Seems natural to suppose”, wrote Mill, “that rules of action must take their whole character … from the end to which they are subservient”. Many moralists have agreed. If we could establish the Summum Bonum, the foundation of morality, the rational basis of moral thinking, this would constitute a criterion, a rule, by means of which men could actually make good practical judgments. This view is radically mistaken. I first try to show this by means of an a priori argument, and then urge the replacement of the “rational model” of practical thinking embodied in this view by an “empirical model”, which should take account of man's actual capacities and limitations. This has important consequences for morality, some of which I investigate.