The natural law theory of ethics has existed in many forms. In its classical form, it is simply the opposite of conventionalist moral scepticism. In its Thomist form, it characterises morality as a function of the rational human nature that God has given us, stressing God’s purposes in Creation as defining our purpose as human beings, and therefore as defining how we ought to lead our lives.
Classical Natural Law Theory
In ancient Greece, conventionalists questioned the weight of the moral law. Moral laws, they suggested, as they vary from nation to nation, must be seen a positive laws, i.e. as laws prescribed by legislative authorities. As such they are mere artefacts of society, conventions, and as such, some of them thought, are not really binding.
This conventionalist view, an early cultural relativism, was opposed from Plato and Aristotle to Cicero and beyond. Morality is not conventional, these thinkers argued, but natural. There is a natural law that must be obeyed whether it is written down by legislative authorities or not. This, what we would now call moral objectivism, is the essence of classical natural law theory.
As conventionalism was associated with the idea that moral laws vary between societies, it is intuitive to see natural law theory as associated with the idea that moral laws are immutable, unchangeable. This association is not necessary, however, and even goes against the views of some natural law theorists, including Aristotle himself.
All that is necessary for a theory to be a natural law theory, in the simple sense introduced so far, is that it oppose conventionalism, that it hold that the moral law is independent of any legislative act. There is no reason, however, why such a moral law must be immutable, and so there is no necessary connection between natural law theory and the view that morality is immutable.
Thomist Natural Law Theory of Neostocism
Thus far, natural law theory has been characterised in quite minimalist terms. More developed natural law theories, however, involve more than the rejection of conventionalism. Indeed, for many, natural law theory is a theory about the relationship between morality and human nature, the theory that who we are determines how we ought to act. There is way of living that is in accordance with human nature, this kind of natural law theory holds, and morality prescribes that we live such a life.
This form of natural law theory is particularly associated with Thomas Aquinas. Aquinas understood human nature to be defined by reason and freedom; it is our ability to reason and to make our own free choices, after all, that sets us apart from animals. Whereas material objects and animals without free will do by nature, deterministically, as God wills them to do, we who have free will may choose either to play our part in God’s plan or not. Reason can tell us what this part is; our purpose is discoverable. With freedom comes responsibility to do as we were made to do.
It is a traditional weakness of natural law theories that they are unclear in their application. It may be that the moral law is independent of any legislative act, but then how are we to know what it requires of us? It may be that there are fundamental principles of action then are evident to reason, but which specific acts do they prescribe in my circumstances? It may be that we ought to act in accordance with our nature, but what is it natural for us to do?