Just my $0.02 worth on natural law, which is yet another entire branch of philosophy. Apart from the concept of God and a God-instilled conscience, or at least some higher Law-Giver, the concept of an immutable natural law seems to me to make little sense. Human societies throughout history have varied widely in their understandings of what was right and what was wrong. It seems to me each society subjectively decides for itself what is “natural” or conveniently redefines the supposedly universal principle (such as what constitutes “murder”) to fit its preferred paradigm. It will not suffice to say, “Oh,
sure, that society condoned what we would call murder, but they knew in their hearts it was wrong” or “Well,
of course they didn’t understand the natural law principle – they were savages.” What sort of natural law would that be? The Aztecs were not savages by a long shot. The basic idea of a natural law apart from God is that it exists Out There, independent of man, waiting for man to discover it. But again, this assumes something that is patently not true – that we are
capable of discovering it and will all
agree on what it is when we do discover it.
For natural law to make sense, it seems to me that it requires a Law-Giver. The basic Christian concept, as I understand it, is that the requirements of the Law are written on the hearts of men regardless of whether they have ever heard of the Ten Commandments and that their God-instilled consciences should convict them. Romans 2:14-15. This makes sense and is indeed talking about an “immutable law” because it flows from God. Whether it is “natural” is debatable. It is “natural” in the sense that man can (supposedly) discover it apart from an explicit knowledge of the Ten Commandments, but its source is God and man’s ability to discover it is God-given.
Another sense in which natural law might make sense is for one to say that evolution has programmed humans with notions of what is right and what is wrong for evolutionary purposes and that the great majority of humans recognize and are guided by these principles. Evolution becomes the Law-Giver. This is one way an atheist can avoid relativism. Evolution is by definition is not immutable, so this is neither “immutable” nor even a “law” in any intelligible sense. It is, however, entirely "natural."
The common claim that “everyone knows murder and rape is wrong” is, I believe, difficult to sustain. Everyone, civilized or not, doesn’t know this. Even highly civilized societies define murder and rape to suit their tastes or purposes and work from that definition. This still might suggest that there is, Out There somewhere, an immutable universal principle that Murder and Rape are wrong and that humans instinctively recognize this but simply adopt flawed notions of the universal principle. Again, this makes sense if we posit a God who has decreed the universal principle, but it really just makes no sense to say, apart from some Law-Giver, that there is a universal principle Out There all humans intuitively recognize.
How would a universal principle exist Out There in the ether, anyway? What would that even
mean? It requires no source? Certainly if we look at the way the natural world, including the animal kingdom, operates, we would have a hard time deriving any principle that rape and murder were wrong. I was reading a fairly high-level discussion on another forum where one of the posters said, "Every time I ask someone to defend the concept of natural law, it always comes back to the Bible or at least God." I think that is probably pretty much true.
Mildly off the topic, but not completely: There has been considerable debate as to whether mathematical principles are "discoverable" (like the conventional notion of natural law) or "invented." A Platonian view would be that they are discoverable. One mathematician, Barry Mazur, has described the Platonian position thusly:
Likewise, it seems to me "natural law" is inevitably theistic.
