"This is a bunch of woo-woo. You're claiming the existence of airy-fairy concepts, which I see no proof of. I believe in Science, and you need to give some evidence that this "natural law" stuff exists. Can you prove it? Can you prove that morality exists, independently of social norms? Can you prove that right and wrong exist prior to us defining them?"
Yes, as a matter of fact, we can.
"Go ahead then."
Simple question: Is child molestation wrong?
"Of course it is."
Why is it wrong?
"Because we have laws against it."
What if we didn't have those "laws" on our books? What if our "lawmakers" never got around to outlawing molestation - or worse, what if there's a law against it now, but they decide to repeal it?
Is child molestation still wrong?
"Yes. There are some things that are just totally wrong, always, no matter what the government thinks, and no matter what society thinks."
So we humans don't create the laws. Because as you just admitted, some laws exist whether we officially "make" them or not.
Child molestation is wrong, and anyone who takes an honest, contemplative look at morality can figure that out, on their own, with or without corresponding social norms. Morality, in this case, stands on its own feet, independently of any human opinion.
Morality is an intrinsic property of Reality.
"But you used a really extreme example. Child molestation? That's extreme! Of course that's always wrong! But when you get into smaller details of law and order, things aren't so black and white. There's a lot of gray area, where some things might be right or wrong, depending on the current norms of the society in question."
Wait a minute. You're overlooking the significance of what just happened. You just admitted that morality can exist independently of human opinion. The example may be extreme, but it still proves the point that morality is objective and built into the basic nature of the universe.
You just admitted that the concept of Natural Law is not woo-woo, but a scientific fact. So you can't use the "woo-woo" argument anymore.
"What you're saying about "morality" is true, but "law" is a different thing. Morality and law are two different things. The former may be an intrinsic property of existence, but the latter is man-made."