This is a review of the book What We Can’t Not Know. that sounds the depths of traditional intellectual inquiry and understanding of the Natural Law.
I would credit Aristotle with the most authoritative seminal effort to delineate and expound on natural law concepts. Since then moral philosophy, in my book, has been footnotes to his dissertations on ethics, morality, civics, politics.
I wold not consider Plato to have made the same contributions to the discussion, but that displays my bias to empirical methods.
I think all the tortured talk about abstract ideas is ignoring the evidence–a higher order of thinking produces language and the derivative abstractions as practical and useful for communicating concepts, analysis and understanding.
Although St. Thomas Aquinas revived discussion of Aristotle in the context of his theological writings, he never lost the thread, so Thomistic thought is Aristotelian with the theological Roman Catholic finish.
As a Jesuit educated boy, particularly in college, I got a serious dunking in Thomistic/Aristotelian Natural Law theory, and other than the concept that St. Thomas created on the soul and mind versus a more materialistic less mystical approach to the human existence, I thought and still think the practical moral and civic virtues hold their own as essential to civil society and moral, ethical behavior.
An atheist can be a moral person, in my opinion. A person who thinks that higher thought and abstract thought can be a product of a more capable brain is not foreclosed from cogent thinking on moral philosophy and ethics. We are not machines or bags of chemicals–there is a complex functionality that creates a higher form of life governed by more than the natural of basic elements, chemical bonds and physical laws.