Query: Why is it that we humans find nothing wrong in defying physical laws, for example by flying, yet we do consider it wrong for us to defy moral laws? Just thinking. Also, doesn’t the fact that some people have defended slavery as "natural" show that we cannot be certain about the content of natural law? (These queries came from an attorney in Jamaica.)  Reply: I’m glad to answer. There are two mistakes in the way you pose your first question. The first mistake is that we don’t defy or violate natural forces; we only make use of them. When an airplane flies, it responds to a number of forces at once. Gravity pulls it down, lift raises it up, and thrust drives it forward. The motion of the airplane is the resultant of all of these forces together.
The other problem is that you are mixing up natural forces with natural laws. A law is a rule and measure of right and wrong, suitable to guide the acts of a free and rational being. From this point of view, “Never murder” is a natural law, but gravity isn’t a law; it is merely a natural force. When we call gravity a “law,” we are speaking analogically, because the airplane is not thinking to itself, “Golly, I am in a gravitational field – I ought to fall down.”
We see then that the original form of the question has things backwards. The airplane cannot defy the force. But a rational being can defy the precept – although it shouldn’t.
As to your second question: Some pre-Civil War American masters defended the enslavement of black Africans on ground that the slaves were naturally inferior. They were gravely mistaken. So doesn’t that show that we can’t be sure about right and wrong?
No, it shows just the opposite. After all, if you weren’t sure that slavery was wrong, then how could you call them mistaken? It is only about real matters of fact that it is possible to make mistakes – and it is only about real matters of fact that it is possible to correct these mistakes.
You wouldn't say that since physicians once mistakenly believed that bloodletting could cure everything from cancer to acne, therefore we can never be sure that it doesn't. Or that since astronomers once mistakenly believed that there were canals on Mars, therefore we can never be sure that they don't exist. Or that since surgeons once operated without washing their hands, therefore we can be sure about the benefits of asepsis. So why should you worry that since some slave-owners once tried to convince themselves that Africans were natural slaves, therefore we can never be sure that they aren't? J. Budziszewski is Professor of Government at the University of Texas at Austin. His latest book is Commentary on Thomas Aquinas’s Treatise on Law (Cambridge University Press, 2014). This article has been republished with permission from his blog The Underground Thomist.