Treatise on Law by Thomas AquinasThis was the first book I completed upon enrolling into Loyola University Chicagos philosophy program, presumably for David Ozars class on ethics. In this class we discussed Natural Law, Deontological and Utilitarian ethical systems, Aquinas being representative of the former. So far as Natural Law was concerned most class discussion concerned the position of the Catholic Church as regards abortion.
Natural Law ethics is rather moribund today thanks to the general acceptance of evolutionary theory. The Thomistic position, however, is that every thing has a nature prefigured in the mind of the creator and that the goal of human behavior ought be to realize the harmonious perfection of these natures.
The only person I have ever met who ostensibly subscribed to Natural Law ethics was Father Piderit, a Jesuit who served for a few disasterously tumultuous years as president of the university.
The Natural Law (Thomas Aquinas, Summa Theologiae, IaIIae, q. 91 and 94) - A Course In Ethics
Eternal and Natural Law: The Foundation of Morals and Law
Whenever someone desires an end, reason commands what is to be done to reach it. For Thomas law is based on community, ordered to the common good. Making law belongs either to everyone or public personages having responsibility for everyone. The leader is obliged to keep common good central when legislating. Corrupt governments are directed to the private good of their leaders.
The term "natural law" is ambiguous. It refers to a type of moral theory, as well as to a type of legal theory, but the core claims of the two kinds of theory are logically independent. It does not refer to the laws of nature , the laws that science aims to describe. According to natural law moral theory, the moral standards that govern human behavior are, in some sense, objectively derived from the nature of human beings and the nature of the world. While being logically independent of natural law legal theory, the two theories intersect.
For moral order to exist, there must be an objective moral law easily perceived, common to all men and obliging to all equally. Without an objective moral law, social order is impossible. Therefore, an objective moral law must exist to guide human behavior, and prevent that individual freedom and the good of society be endangered. In that case, morality would depend entirely on the number of policemen, and each man would need a policeman to watch him a tall times. Let us consider another point. All law is a manifestation of the will of a legislator who imposes, commands, forbids, permits and punishes. Since we all have the same nature, the will of any man is equal to that of another, and no one man can impose his will on another.
Treatise on Law is Thomas Aquinas ' major work of legal philosophy. Along with Aristotelianism , it forms the basis for the legal theory of Catholic canon law. Aquinas defines a law as "an ordinance of reason for the common good, made by him who has care of the community, and promulgated. Law is an ordinance of reason because it must be reasonable  or based in reason and not merely in the will of the legislator. It is promulgated so that the law can be known. Thus from the four preceding articles [of Question 90], the definition of law may be gathered; and it is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated. Strictly speaking, this is a definition of human law.