Natural law, according to Thomas Aquinas, has its foundation in the evidence and operation of natural, human reason. Its primary precepts are self-evident. Awareness of these precepts does not presuppose knowledge of, or even belief in, the existence of God. The most interesting criticisms of Thomas Aquinas's natural-law teaching in modern times have been advanced by the political philosopher Leo Strauss and his followers. The purpose of this book is to show that these criticisms are based on misunderstandings and that they are inconclusive at best. Thomas Aquinas's natural-law teaching is fully rational. It is accessible to man as man.