In Philip L. Quinn and Charles Taliaferro (eds.), A Companion to the Philosophy of Religion (Blackwell Publishers, 1996).
Natural law is the body of moral norms and other practical principles which provide reasons (including moral reasons) for action and restraint.
The most basic precepts of natural law direct people to choose and act for intelligible ends and purposes. These precepts, which Thomas Aquinas called “ the first principles of practical reason,” refer to the range of “ basic ” (i.e., non – instrumental or not – merely – instrumental) human goods for the sake of which people can intelligently act. Insofar as a possible action promises to instantiate at least one such good, performing it has an intelligible point.
However, the diversity of goods which provide non – instrumental practical reasons, together with the range of subrational factors which can motivate people to act in ways contrary to the prescriptions of practical reason, make it unavoidable that people will face morally signifi cant free choices. Moral norms, including such very general moral principles as the golden rule of fairness and the Pauline principle that evil may not be done even to achieve good consequences, guide choice in such circumstances (though they do not always narrow the range of fully reasonable, morally good options to one) by providing conclusive reasons to choose certain options and to refrain from choosing others. Moral norms are needed in addition to the most basic practical principles because the latter exclude only those possibilities for choice which lack an intelligible point (and, as such, are the objects of merely emotional as opposed to rational motivation).