A teleological moral theory, founded on an anthropology of the human person (usually philosophical, theological, or both), that specifies a conception of the good and includes absolute principles of prohibition. Because this theory includes some absolute negative precepts or rules (e.g., the prohibition against directly killing innocent human life), some contemporary scholars characterize it as a "mixed teleological-deontological" method. This description, however, fails to draw the fine but essential distinctions between consequentialism and teleology. Natural Law is better thought of as a broader methodological approach that encompasses essential elements of both consequentialism and deontology, but is a third and altogether distinct ethical methodology.
Though it is often assumed that natural law refers to what nature dictates, as in the "laws of nature," natural law more accurately refers to what reason dictates. While St. Thomas’s conception of natural law relies on revealed sources of Faith from Scripture and the lived Tradition of the Church, it is based primarily on rational arguments. What significance one assigns to "reason" will to a large degree determine what type of natural law theory one holds. For example, Cicero (d. 43 BC) maintained that the natural law consisted of innate rules of reason to which all actions should conform. The Stoic Ulpian (d. 228 AD), on the other hand, down played the role of reason and held that natural law referred to the dispositions human beings share with other animals. In the Summa Theologica I-II, Question 94, Article 2, St. Thomas appears to incorporate both notions of natural law, but not as two competing theories. Rather, all the aspects of human nature are understood to be revealed by God and discernable through human reason. In the first place, he says that the precepts of the natural law correspond to practical reason: "all other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man’s good (or evil) belongs to the precepts of the natural law as something to be done or avoided" (ST I-II, Q. 94, Art. 2). He then says that man has a natural inclination in three senses. At the most basic level, in accord with all living things, whatever is a means of preserving human life and warding off its obstacles belongs to the natural law. At the next higher level, St. Thomas speaks of man’s inclinations according to that nature which he has in common with other animals, that is, that "which nature has taught to all animals, such as sexual intercourse, education of offspring and so forth." Third, he says that at the highest level there is an inclination to good that is in according to the nature of man’s reason:
"Thus man has a natural inclination to know the truth about God, and to live in society: and in this respect, whatever pertains to this inclination belongs to the natural law; for instance, to shun ignorance, to avoid offending those among whom one has to live…."