St. Thomas Aquinas on the Natural Law.
Aquinas bases his doctrine on the natural law, as one would expect, 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). In 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 reason or intellect of the ruler. It is the idea of what should be done to ensure the well-ordered functioning of whatever community the ruler has care 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. ST 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 there is in us a natural law. 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 that are ruled by that Law 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 reflect the order by which God directs them through their nature for their own benefit, reflects the Eternal Law in their own natures. (S.T. I-IIae, 91, 2)
The Natural Law, as applied to the case of human beings, requires greater precision because of the fact that we have reason and free will. It is our nature as 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 human beings must exercise our natural reason to discover what is best for us in order to achieve the end to which their nature inclines. Furthermore, we must exercise our freedom, by choosing what reason determines to naturally suited to us, i.e. what is best for our nature. The natural inclination of humans to achieve their proper end through reason and free will is the natural law. Formally defined, the Natural Law is humans’ participation in the Eternal Law, through reason and will. Humans 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.
In applying this universal notion of Natural Law to the human person, one first must decide what it is that God has ordained human nature to be inclined toward. Since each thing has a nature given it by God, and each thing has a natural end, so there is a fulfillment to human activity of living. When a person discovers by reason what the purpose of living is, he or she discover his or her natural end is. Accepting the medieval dictum “happiness is what all desire” a person 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, so much so that 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 that is a definition of an apparent good. Thus, this position of Aquinas has a certain phenomenological appeal: a person does anything and everything he or she does only because that thing at least “appears” to be good. Even when I choose something that I know is bad for myself, I nevertheless chooses it under some aspect of good, i.e., as some kind of good. I know the cake is fattening, for example, and I don’t choose to eat it as fattening. I do, however, choose to eat it as tasty (which is an apparent, though not a true, good).
On the level that we share with all substances, the Natural Law commands that we preserve ourselves in being. Therefore, one of the most basic precepts of the Natural Law is to not commit suicide. (Nevertheless, suicide can, sadly, be chosen as an apparent good, e.g., as the cessation of pain.) On the level we share with all living things, the Natural Law commands that we take care of our life, and transmit that 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, the fulfillment of the Natural Law consists in the exercise those activities that are unique of humans, i.e., knowledge and love, and in a state that is also natural to human persons, i.e., society. The Natural Law, thus, 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 those things that make for the harmonious functioning of society (“Thou shalt not kill,” “Thou shalt not steal.”) Human nature also shows that each of 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 so far outlined are only the most basic. “The good is to be done and pursued and evil is to be avoided” is not very helpful for making actual choices. Therefore, Aquinas believes that one needs one’s reason to be perfected by the virtues, especially prudence, in order 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, then, in a quite 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 by which 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 one gets to the proximate level of commanding or forbidding specific acts. It grounds the notion that there are some things that are wrong, always and everywhere, i.e., “crimes against humanity,” while avoiding the obvious difficulties of claiming that 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 what in particular the Natural Law requires.