Lately, I have found myself attracted to moral nihilism, as espoused by Mackie and Joyce. I define moral nihilism as the denial of the existence of objective moral truth. However, as moral nihilism is both counterintuitive and uncomfortable, I would like to ask whether any of you are aware of any convincing arguments that refute this position. As I am an atheist, please keep in mind that any arguments that invoke God would not be convincing. In the following, I will summarize the case for moral nihilism:
The only evidence for the existence of objective moral truth is our so-called moral intuition. However, this intuition is unreliable, and it may be alternatively explained by natural selection. It is unreliable because there is wide disagreement over the actual content of objective moral truth (Mackie's "argument from relativity"). It may be explained by natural selection by noting that empathy (the basis, in my view, of moral intuition) would likely promote group coherence and cooperation for mutual benefit, thereby enabling those with empathy to survive and reproduce more successfully than those who are non-empathetic. Furthermore, if objective moral truth existed, it would be a strange metaphysical entity indeed (Mackie's "argument from queerness"), as it would be intrinsically motivating, yet not confirmable by sense-data. Therefore, applying Occam's Razor, one should not posit such a strange entity if the only evidence for it (so-called moral intuition) is unreliable and may be explained instead by a scientific theory (natural selection) that is widely accepted.
I have a few questions here.
#1. Why do you assume that morality is a product of intuitions?
#2. Why do you assume that moral truths cannot be confirmed through sense-experience?
Also, later on in your post you consider a person who wants to torture a child and believes he will not be caught if he does so. You want to know how to convince him not to torture the child. My question is why do you begin with this example? Suppose we begin with this example instead:
"There is a person who wants to stab his own eye out with a knife. What could we say to convince him not to? Is there any empirical evidence that he should not stab his own eye out? Or is it just a matter of opinion that he should not do so?"
Some will say that this example misses the point. If the guy wants to stab his own eye out that is not a moral issue but just a personal decision of his. But is it? Are there not some grounds for saying that certain actions are irrational and SHOULDN'T BE DONE regardless of whether they harm other people or not? And if so, how does one make sense of moral nihilism, of the idea that we cannot say what a person "should" or "should not" do?
To sum up this point, I think that you are on the wrong track in the way that you are posing the question of moral nihilism. The first thing we need to get clear is the question of what morality is and why we are thought to need it. Once we get that clear we can then decide whether it really is necessary to human life or if it is instead just a superstitious remnant left over from religion.