What We Cannot Not Know

While initially it may seem to be an impractical subject, the concept of the natural law has profound implications on every area of the intellectual life. It is well worth the concentration necessary to familiarize oneself with its basic principles.

A very good summary of St. Thomas Aquinas's view of the nature of law may be found here. There is much more to his thought, but this link is a good summation that provides a glimpse of the internal coherence of Aquinas's thought on the subject. For a good introduction to Aquinas's view of law, Professor Jay Budziszewski's Written on the Heart: The Case for Natural Law is excellent. Upon completion of that book, Professor Budziszewski's What We Can't Not Know: A Guide is a good choice (even as an initial introduction to the subject). While scholarly and precise, these two books make the subject of natural law accessible to people who do not have a philosophical or legal background. Another very helpful book that illustrates the natural law is C.S. Lewis's The Abolition of Man.