Modern society is riven by social divisions: between conservatives and progressives; liberals and socialists; the mainstream and the rise of far-right political groups etc. Instead of truth, there are ‘post-truth’ and ‘alternative facts’. In the wake of problems caused by untruthful politicians and world leaders, by Brexit and Covid, the need to repair or rebuild our communities has become paramount, but what kind of community should we build, and on what foundations? This book suggests that natural law is such a foundation. Natural Law and Modern Society presents a new theory of natural law, grounded in the thought of Saint Thomas Aquinas, aimed at answering questions relevant to the world of today: from the nature of morality and ethics to the theory of law, obligation and political authority; from the domestic realm to international community. It seeks to elicit from the natural law tradition timeless truths concerning the human condition, in particular the social and political dimensions to human existence. This mode of existence, it argues, is not a problem to be resolved through some permutation of political institutions, but a predicament to be managed. At the heart of the book is the identification of a ‘core morality’: a set of moral requirements that are foundational to every society at all places and times, as distinct from those standards that are particular to this or that society at some time.