For as long as there has been human morality, questions have risen about “natural law,” What is it? How do we know we have it? Is natural law the same for everyone? Every nation? Every religion? Is natural law God’s voice in each of us? Does Natural Law change? How can that be? There appears to be a soft consensus that all human beings have at least one thing in common, they are more or less rational or reasonable. Beyond that there is much uncertainty, disagreement, aggravation and hostility. Like music, morality can be seen as structured or free-flowing, it can be self-evident or impossible to comprehend, it can be the unifying force of human history or the cause of endless hostility. But, the suspicion or hope endures that when humans act more or less rationally they act more or less according to their nature.

Thomas Aquinas, one of the most persuasive exponents of natural law morality is often misrepresented as holding that natural law is a set of rigid and inflexible self-evident principles. There is another view of Aquinas’ insight into human morality which acknowledges that the universe in which humans learn to function is more or less contingent, sometime orderly, sometimes chaotic. The natural world does not always work perfectly, sometime reversing itself, frustrating its own purposes, and sometimes making it difficult or impossible for the human to be certain which principles apply to which facts and when. This essay introduces the reader to this view of a changeable natural law, which does not deny the validity of moral principles but affirms their applicability to a very imperfect and changing natural world.

Originally published 1965; Revised 2012.