When I was sixteen years old I traveled to Europe with a group of Christian, homeschooled young people. Bright, well-read teens with a desire for knowledge and the ability to converse easily with adults.
Our hired tour guide for the trip was a very kind British man, who happened to be an Atheist. To his eternal credit, he graciously allowed us to debate with him on matters of theology, evolution and morality as much as we wished (in fact I missed watching the train go under the English Channel because I was absorbed in one of these debates, but that's another story). We covered a lot of topics, but the one that will probably remain with me the longest was one that took place in the Paris train station. We were sitting on plastic chairs, waiting for our signal to go, and somehow the subject of morality came up.
Our point was simply this. If you put God out of the equation, who determines what is right or wrong? For instance, there would be no reason why I couldn't go up and murder whomever I pleased. Our guide's answer was that murder was still wrong because the government decreed it so.
I think our tour guide ought to read Lewis.
In Chapter Three, Lewis discusses the difference between scientific laws and the Moral Law.
“There [are] two odd things about the human race. First, that they [are] haunted by the idea of a sort of behaviour they ought to practise, what you might call fair play, or decency, or morality, or the Law of Nature. Second, that they did not in fact do so.”
He goes on to explain that a scientific law is a law of what happens, what must always happen in certain situations. However, humans do not always follow the Moral Law even though they all know it exists.
“The law of gravity tells you what stones do if you drop them; but the Law of Human Nature tells you what human beings ought to do and do not.”
This isn't a law that we make up, it's a law that we know, yet it's not a law we always follow. It's not scientific, it's not cultural... it's an expectation that we're inborn with.
“If we ask: 'Why ought I to be unselfish?' and you reply 'Because it is good for society,' we may then ask, 'Why should I care what's good for society except what happens to pay me personally?' and then you will have to say, 'Because you ought to be unselfish' – which simply brings us back to where we started.”
An unwinnable argument? Only if there is not something more... which we have already extablished that there is. The Moral Law exists and it tells us that we ought to be unselfish.
Where does this law come from?