One of the more fruitful ideas to emerge in Western history is the concept of natural law. I’m not talking about the laws of physics and chemistry, but, rather, the fundamental moral truths that are, to some extent, knowable by everyone everywhere, at all times. C.S. Lewis gave examples of these truths, common to all the world’s great religions, at the end of his book The Abolition of Man. The American Founders took the natural law for granted. Even the most strongly deistic, such as Thomas Jefferson, appealed to it.
Unfortunately, the idea fell on hard times in the twentieth century. Some Christians thought (mistakenly) that the concept was unbiblical. And many secular intellectuals came to doubt that moral claims had truth value at all. When I was in college in the 1980s, relativism was the fashion, and Lewis’ book was the only source I knew of that defended natural law against its modern detractors.
Fortunately, there are now a number of first-rate scholars defending and developing natural law theory. Unfortunately, the discussion is pretty abstract and is, at times, contentious. If you were to read only one book explaining and defending natural law, however, I would recommend J. Budziszewski’s What We Can’t Not Know: A Guide. Spence Publishing originally released the book in 2003, and I recently re-read it. It’s extremely lucid, irenic, and crisply written. I’ve put it on the list of books my children must read before they graduate from high school.
In fact, I wish everyone would read the book, and regret that it was not more widely discussed when it was released. So I’m pleased to see that Ignatius Books has just released a revised and expanded edition in paperback and electronic formats. Here’s their description:

In this new revised edition of his groundbreaking work, Professor J. Budziszewski questions the modern assumption that moral truths are unknowable. With clear and logical arguments he rehabilitates the natural law tradition and restores confidence in a moral code based upon human nature.
What We Can’t Not Know explains the rational foundation of what we all really know to be right and wrong and shows how that foundation has been kicked out from under western society. Having gone through stages of atheism and nihilism in his own search for truth, Budziszewski understands the philosophical and personal roots of moral relativism. With wisdom born of both experience and rigorous intellectual inquiry, he offers a firm foothold to those who are attempting either to understand or to defend the reasonableness of traditional morality.
While natural law bridges the chasms that can be caused by religious and philosophical differences, Budziszewski believes that natural law theory has entered a new phase, in which theology will again have pride of place. While religious belief might appear to hamper the search for common ground, Budziszewski demonstrates that it is not an obstacle, but a pathway to apprehending universal norms of behavior.

Also check out this new interview with Budziszewski at Breakpoint.
Natural law theory actually has some interesting affinities with ID, which Budziszewski mentions in the book. In any case, if you haven’t yet read What We Can’t Not Know, now’s your chance.