Saint Thomas Aquinas offers a theory of natural law that is rooted in eternal and divine law. Notice again how Aquinas' natural law differs from consequentialism. Do you think there are any eternal natural laws?
St. Thomas Aquinas bases his doctrine for the natural law, on his understanding of God and his relation to his creation. He grounds his theory of natural law in the notion of an eternal law (in God). When asking whether there is an eternal law, he begins by stating a general definition of all law: Law is a dictate of reason from the ruler for the community he rules. This dictate of reason is first and foremost within the ruler's reason or intellect.
It is the idea of what should be done to insure the well-ordered functioning of the community the ruler cares for. It is a fundamental tenet of Aquinas' political theory that rulers rule for the sake of the governed, i.e. for the good and well-being of those subject to the ruler. Since he has elsewhere shown that God rules the world with his reason, since he is the cause of its being (cf. Summa Theologiae Ia 22, 1-2), Aquinas concludes that God has in his intellect an idea by which he governs the world. This idea, in God, for the governance of things is the eternal law. (Summa Theologiae I-IIae, 91, 1)
Next, Aquinas asks whether a natural law exists. First, he makes a distinction: A law is not only in the reason of a ruler, but may also be in the thing that is ruled. In the case of the eternal law, the things of creation the law rules have it imprinted on the them through their nature or essence. Since things act according to their nature, they derive their proper acts and ends (final cause) according to the law that is written into their nature. Everything in nature, insofar as they reflects the order by which God directs them through their nature for their own benefit, reflects the eternal law in their own natures. (Summa Theologia I-IIae, 91, 2)
The natural law, in case of human beings, requires greater precision due to the fact that we have reason and free will. It is the our nature humans to act freely (i.e. to be provident for ourselves and others) by being inclined toward our proper acts and end. That is, we must exercise our natural reason to discover what is best for us to achieve the end to which their nature inclines. Furthermore, we must exercise our freedom, by choosing what reason determines to naturally suit us, i.e. what is best for our nature.
People are naturally inclined to achieve their proper end through reason and free will, according to the natural law. Formally defined, the natural law is humans' participation in the eternal law, through reason and will. People actively participate in the eternal law of God (the governance of the world) by using reason in conformity with the natural law to discern what is good and evil.
When applying this universal notion of natural law to people, we must first decide what God has ordained human nature is inclined toward. Since each thing has a nature, which God has given, and each thing has a natural end, there is a fulfillment to living. When someone discovers by reason what the purpose of living is, he or she discovers his or her natural end. Accepting the medieval dictum "happiness is what all desire" someone is happy when he or she achieves this natural end.
Aquinas distinguishes different levels of precepts or commands that the Natural Law entails. The most universal is the command, "good is to be done and pursued and evil avoided." This applies to everything and everyone, some consider it to be more of a description or definition of what we mean by "good."
For these philosophers, a thing is "good" just in case it is pursued or done by someone. Aquinas would agree with this to a certain extent; but he would say that is a definition of an apparent good. Aquinas's position has a certain phenomenological appeal: someone does anything and everything he or she does only because it "appears" to be good. Even when I choose something I know is bad for myself, I choose it nevertheless under some aspect of good, i.e. as some kind of good. Fir example, I know cake is unhealthy, and I don't choose to eat it as unhealthy. I do, however, choose to eat it because it is tasty (which is an apparent, though not a true, good).
On the level we share with all substances, natural law commands we preserve ourselves in being. One of the most basic precepts of the natural law is to not commit suicide. (Nevertheless, we can choose suicide as an apparent good, to end pain.)
On the level we share with all living things, natural law commands that we take care of our life, and transmit life to the next generation. Thus, almost as basic as the preservation of our lives, the natural law commands us to rear and care for offspring.
On the level that is most specific to humans, natural law commands us to exercise those activities that are unique of humans, i.e. knowledge and love, in a state that is natural to people, i.e. society. Natural law, commands us to develop our rational and moral capacities by growing in the virtues of intellect (prudence, art, and science) and will (justice, courage, temperance). Natural law also commands us to make a harmonious, functioning society ("Thou shalt not kill," "Thou shalt not steal.") Human nature also shows us have a destiny beyond this world, too. Man's infinite capacity to know and love shows that he is destined to know and love an infinite being, God.
All of these levels of precepts are the most basic. "The good is to be done and pursued and evil is to be avoided" is not helpful for making actual choices. Therefore, Aquinas believes we need to perfect our reason by the virtues, especially prudence, to discover precepts of the natural law that are more proximate to the choices that one has to make on a day-to-day basis.
The Thomistic notion of natural law has its roots, in a basic understanding of the universe as caused and cared for by God, and the basic notion of what a law is. It is a fairly sophisticated notion to ground the legitimacy of human law in something more universal than the mere agreement and decree of legislators. Yet, it allows that what the natural law commands or allows is not perfectly obvious when we get to the proximate level of commanding or forbidding specific acts. It grounds the notion that some things that are wrong, always and everywhere, i.e. "crimes against humanity," while avoiding the obvious difficulties of claiming this is determined by any sort of human consensus. Nevertheless, it still sees the interplay of people in social and rational discourse as necessary to determine exactly what the natural law requires.