Natural theology and natural law › What makes something a law?

In I-II, question 90, Aquinas defines law through reason, the common good, appropriate authority, and promulgation. The account does not identify law with any command issued by a powerful person. It asks who is responsible for the community and how a rule becomes publicly available as a guide for action.

Question 91 distinguishes eternal, natural, human, and divine law. Eternal law concerns divine governance. Natural law is the rational creature's participation in that governance (I-II q.91 a.2). Human law supplies determinations appropriate to human communities, while divine law directs human beings through revelation.

Participation needs explanation here. A rational creature does not merely undergo an external regularity. Human beings can deliberate about what they should pursue and can fail to act in accordance with that judgment. Natural law names practical principles available to reason, within Aquinas's theological account of why those principles have authority.

The framework consequently connects theology and ethics without requiring a separate revelation for every ordinary moral judgment. Yet it does not follow that every person recognizes every moral conclusion equally well. Question 94 explicitly distinguishes common principles from more particular conclusions and their variable application.