Natural law ethicists are kept constantly squirming between the underlying idea that what is natural is good, and the need to make some ethical distinctions between different forms of behavior that are, in biological terms, natural to human beings. This wasn’t an insoluble problem for Aristotle, who believed that everything in the universe exists for a purpose, and has a nature conducive to that purpose. Just as the purpose of a knife is to cut, and so a good knife is a sharp one, so Aristotle seems to have thought that human beings exist for a purpose, and their nature accords with their purpose. But knives have creators, and, unless we assume a divine creation, human beings do not. For the substantial proportion of natural law theorists who work within the Roman Catholic tradition, the assumption of a divine creator poses no problem. But to the others, and indeed to anyone who has accepts a modern scientific view of our origins, the problem is insoluble, for evolutionary theory breaks the link between what is natural and what is good. Nature, understood in evolutionary terms, carries no moral value.
Peter Singer, ‘A Reply to Martha Nussbaum’, The Tanner Lectures on Human Values, 2002