How can 'Natural Moral Law' be critiqued as an ethical theory?

‘Natural Moral Law’ was perhaps most famously developed by St. Thomas Aquinas, in his theological guide ‘Summa Theologica’. Aquinas’ ‘Natural Moral Law’ is an absolutist and deontological ethical approach – meaning it prescribes fixed rules of morality. Believing that it was human nature to pursue good and avoid evil, Aquinas defined five ‘Primary Precepts’ which detail the steps of leading a moral life. Respectively, the precepts are: to seek self-preservation; to reproduce; to nurture offspring through education; to live in society; and to worship God. In this regard, ‘Natural Moral Law’ is a source of clear values, which appear simple to adhere to and objective in nature. However, from a relativist perspective there is no singular ‘moral law’ agreed on by humanity. In ‘Elements of Moral Philosophy’ Rachels observes that Natural Law is based upon “a certain view of what the world is like”. As a primarily Christian theory, it fails to acknowledge that the world is actually full of different moralities, and in turn that the ‘Primary Precepts’ are not considered ethically important in all cultures. Furthermore, if we assume that the precepts absolutely describe how to be morally ‘good’, then we imply those without children (or who cannot conceive children) can never lead a truly moral life - as two of the precepts cannot be fulfilled. Thus, ‘Natural Moral Law’ can be critiqued for being exclusive, as it is often difficult to subjectively apply outside a Christian context.                                                                                                                   On the contrary, supporters of ‘Natural Moral Law’ refute this claim. Aquinas proposed that the ‘moral law’ is perceived through revelation (the word of God) and through human reason – both of which must find harmony with one another. The emphasis on a human rationality shared by all allows ‘Natural Moral Law’ to transcend cultural differences, and instead focus on common ethical ideas, which – theoretically – can be concluded by everyone. Consequently, ‘Natural Moral Law’ can be seen as an ethical theory which is universal and accessible in nature. Nonetheless, critics of ‘Natural Moral Law’ argue there is no such ‘essential human nature’ or ‘shared rationality’. Philosopher Kai Neilsen contends that scientifically, there is no justification for the belief that humans have the same natural inclinations when it comes to ethics. This is an opinion shared by Rachels, who stated that “the view of the world on which [Natural Moral Law] rests is out of keeping with modern science”. Assuming, then, that there is no mutual ethical reasoning which is naturally apparent in humans, a main premise of Aquinas’ theory appears to be unsound – in both a scientific and a theological sense. It is worth noting that scientific advancements and cultural understandings have surely evolved beyond anything Aquinas could have imagined when designing ‘Natural Moral Law’ in the 13^th century. However, the suggestion that science may be used to discredit the foundations of ‘Natural Moral Law’ implies that it is an outdated theory, which perhaps cannot be appropriated in the 21^st century.