Thomas Aquinas's Aristotelian interpretation of natural law has shaped western law and politics, although it is a minor section in the Summa Theologiae (ST II.I.94). It belongs within a comprehensive account of four levels of law (ST II.I.90-104). Eternal law is incomprehensible to us, because it is the order upon which all other order depends. We cannot think outside the laws we think with. Divine law is revealed in scripture and is meaningful only to those who accept scriptural authority. Natural law is what we have in common. It refers to our rational capacity to discern general principles in the order of nature to enable us to flourish as a species in communities, given that by nature we are social animals. Today, we might say that it is in our DNA.
Human law is the interpretation of natural law in different contexts (ST II.I.95-97). Like Aristotle, Aquinas believed that just laws relate to the species, so the collective good comes before the individual good – although in a just society, these are not in conflict. This means that law is not about individual morality, and individual vices should only be legislated against when they threaten harm to others. Unlike Aristotle, Aquinas believed that an informed conscience takes precedence over law. No individual should obey a law that he or she believes to be unjust, because laws that violate reason are not laws. Moreover, laws must have sufficient flexibility to be waived when necessary in the interests of the common good.
Natural law supports different cultures and religions, but unjust societies are those whose laws violate natural law.
Modern thinkers who appeal to natural law as a foundation for morality often lose sight of Aquinas's naturalism, presenting it as a transcendent rational capacity or divine command that overrides our natural instincts and desires. This manifests itself in the rationalist quest to conquer nature (now redounding on us in a looming environmental catastrophe), and in the Catholic church's attempt to use politics and law to impose its views on sexuality over and against changing social customs.
Aquinas argues that laws should change to reflect customs (although custom cannot change natural or divine law). I'll focus on two issues relating to this in terms of a widening gulf between the Catholic hierarchy and modern culture, including many Catholics.
Contraception: Aquinas believed that the sex act must be intended for procreation for the preservation of the species (ST II.II.153.2). He also believed that children need to be raised in a loving environment, and marriage is the proper context for this. But we now know that females are not always fertile, and sexual activity among animals seems to be less functional than he realised. The changing role of women is also a transformation in culture and custom that requires a radical rethinking of law and reproductive ethics. The prohibition of artificial birth control finds little support from this reading of natural law, particularly since it flies in the face of customary practice among many Catholics and non-Catholics.
Homosexuality: once one accepts that non-procreative, loving sex is good, the argument from natural law against homosexuality becomes untenable. What remains central from a Thomist perspective is what it means to live well as a sexual creature whose relationships reflect the love of God and respect the dignity of the human made in the image of God.
Natural law is our rational capacity to interpret the laws of nature in order to use our scientific knowledge well. It is still relevant, even if our science is very different from Aquinas's, as we see from debates about the ethical implications of the laws of evolution. The Darwinian eugenicists of the early 20th century were engaged in one kind of natural law deriving from evolutionary science, and debates on this blog about genetic altruism are another form of natural law. They follow Aquinas insofar as they express a desire to discern order and goodness rather than randomness and futility in what science reveals to us about nature. Evolutionary eugenics may be as rationally defensible as evolutionary altruism, so why do we think one is bad and the other is good? Aquinas would have said because one respects the dignity of the human made in the image of God and the other violates it, but without that perspective, the answer is less clear.