One problem I find with Aquinas’s proposition that natural law must serve as a basis for positive law is his presupposition of the existence of an objective justice.
Aquinas recognizes two components active in the development of the natural law: the fact that God has an “Eternal Reason” and that humans, as “rational creature[s],” have a share of this. It follows that the natural law is “this participation of the eternal law in the rational creature.” Which is to say, natural law is what we rationally understand about divine law. Aquinas’s practical application of the natural law is as a standard to which we must hold our laws. He presents Augustine’s claim that “’that which is not just seems to be no law at all,’ … but a perversion of law.” Justice is defined here as being derived from natural law. Practically, if I find a law of my country to be unjust according to my understanding of natural law, can I break it with the confidence that I will not suffer the consequences I would for breaking a just law? In what court can I expect to be given protection from those who wrote the law and want to enforce it? A divine court? It’s hard to have confidence in a court with no precedent. The disparity between Aquinas’s argument in speculative reason and mine in practical reason is that I assumed the existence of subjective justice. So, let’s assume the opposite: there exists only an objective justice.
In order for justice to be objective, then natural law must be static. However, if natural law is based on our understanding of the divine law, then how can it remain the same? No doubt, Aquinas believes that we as a species we are more knowledgeable than our distance ancestors, and we continue to learn more about the eternal law as we grow, reproduce, and evolve. But if we are to evolve, then it must follow that natural law too must evolve and change as our understanding of the eternal law expands and becomes clearer. Just as mutation, a seemingly unnatural deviation from the norm is the first step in the growth of a species toward survival or evolution, so deviation from the natural law must occur in order to reach a rational understanding that adheres more closely to the eternal law. Here is the contradiction in Aquinas’s reasoning, for if natural law is not static there must be no objective justice. Without objective justice, natural law has no practical application.
Aquinas’s evaluation of the validity of laws based on their justness assumes that laws draw their power from this justice, or form the natural law. As this stands firm with his “speculative reason,” this may be an ideal to aspire to, but it is not empirically true. Laws in their most basic form are contracts between people. Any contract only has as much power as the ability of either party to enforce the punishments laid out within the contract upon the other party in the instance of their breach of the contracts terms. In the same manner, any law only has as much power as the ability of those who support the law to enforce it. Thusly, rather than judge positive law by what we might hold to be natural law, it is more practical to judge it by the benefit it serves those parties involved, or, in Aquinas’s words, by its service to the common good, or the good of all parties affected by the law. Whether this common good is easily defined is yet another question for another time.