Natural theology and natural law › Practical reason and natural inclinations

Aquinas's first practical principle directs us to pursue good and avoid evil (I-II q.94 a.2). It does not by itself supply a list of detailed statutes. Further reasoning concerns the goods that practical reason apprehends in relation to human inclinations.

His account identifies preservation of life, matters shared with other animals such as generation and education of offspring, and activities belonging to rational social life, including seeking truth about God and living with others. These are ordered sources of practical precepts, rather than a report that every desire is morally authoritative.

The distinction is explicit in question 94, article 4, reply 3: inclinations of the other powers should be directed according to reason. An impulse cannot justify itself merely by being natural in the sense of occurring spontaneously. The theory asks what contributes to a rational human life, which requires assessing desires as well as acknowledging them.

A reader can challenge the proposed account of human nature or the move from its inclinations to obligations. Those challenges concern substantive premises in the theory. They are stronger when they address Aquinas's practical reasoning than when they assume that he simply derives a moral rule from any observed behavior.