 Aquinas takes this to be self-evident, because he accepts Aristotle’s claim at the start of the Nicomachean Ethics that the good is what all things desire. By beginning here, Aquinas builds into the ground floor of his account a substantive story about  moral motivation.  The story requires tying the ensuing development of his theory to facts about what human beings actually desire; without that tie, the theory would equivocate on what it means by the good. Aquinas explicitly recognizes as much, remarking that “the order of the precepts of natural law accords with the order of natural inclinations” (ST 1a2ae 94.2c). At this point, Aquinas could appeal to eudaimonism: that all human beings desire happiness as their ultimate end (§8.1). A very thin conception of natural law might suppose that human beings are given only this much innate direction, and thereafter left to sort out for themselves what will make them happy. But Aquinas’s conception of natural law is much thicker, both in the sense that he articulates a rich and substantive notion of the happiness that is our ultimate end (as above), and in the sense that he thinks we have been given various further innate inclinations, intended to provide us with specific guidance toward that ultimate end. Among these are inclinations to preserve our own lives, toward sexual activity, toward educating the young, toward knowing the truth about God, and toward living in society (ST 1a2ae 94.2c; see also SCG III.129). These innate inclinations, combined with the first practical principle (“The good should be done…”) and Aquinas’s substantive conception of happiness, are the foundations from which arises a comprehensive account of the moral law, relying on  conscience  as the rational activity of working out what ought to be done from a moral point of view (ST 1a 79.13).