Does natural law presupposes faith? Those who reject natural law often say it does. “It’s a Christian thing,” they say. “You only believe in it because you’re Christian.”
No. The foundational principles of natural law, for example that good is to be done and pursued, that evil is to be avoided, and that we must never do evil so that good will result – and the easier corollaries, for example that we should honor our parents and that we should never deliberately take innocent human life -- are not only right for everyone but at some level known to everyone. As St. Paul says, they are written on the heart. Even atheists know them. In this sense, natural law is common ground for all human beings. So it shouldn’t surprise us that under one name or another, the idea keeps coming up among Christians, Jews, Muslims, Confucians, and all sorts of people, though of course some of these traditions have more resources for discussing natural law than others do.
Yet the fact that natural law is a common ground doesn’t mean that everyone is standing on it. I might know something but not realize that I know it. I might know it, but violate it. I might know it, but carve out self-serving exceptions for myself. I might know it, but fail to work out its implications. Or I might know it, but deny that I know it -- I tell myself that I don’t know what I really do know.
Friends of natural law sometimes overlook these difficulties. Opponents of natural law sometimes try to cash in on them. “You say some truths are evident in themselves? Well, they aren’t evident to me. Like when you say we shouldn’t do evil for the sake of good. What I say is, sometimes we have to do evil to keep something worse from happening.”
For all these reasons, the common ground is a slippery common ground, hard to stand on, wet with the dew of our evasions.
This gives us a reason to revisit the question of whether natural law presupposes faith. In principle, no, because we don’t require faith to know its foundational principles. But in practice, sometimes yes, because we might require faith to bring ourselves to admit that we know them. If I deny God, then the law is still written on my heart, but I may deny that it is law, claiming for example that it’s just a primate inhibition. If I deny the possibility of divine forgiveness, then my conscience stills speaks to me, but I may refuse to listen, claiming for example that it’s just a neurotic hangup. For that matter, I may not even want to be forgiven, because I would have to change. I may prefer my vice to the possibility of innocence and freedom. So I avert my eyes from the inscription on my heart; I try to rub it out; failing that, I cover it up with a shroud of rationalizations.
The upshot is that although the ancient writers were right to say that the natural knowledge of the reality of God and of His law is a praeambulus fidei, a preamble to faith, the relationship works in the other direction too. For many of us, faith may be a praeambula amicitiae cum natura, a preamble to friendship with our natural knowledge.