I've been following Sam Harris for a while and have recently read The Moral Landscape in which he attempts to answer this question. As an atheist, my own view is that there can be no objective basis to morality and so morality must be a function of culture and convention only, however Harris valiantly argues against this popular point of view in his book.
Reading Sam Harris has been illuminating - while I still don't really believe in the fundamental objectivity of morality, his arguments help to explain how one might apply science, logic and reason to compare alternative moral values and determine which are to be preferred.
Before delving into Harris's view of morality, and my own, I'll be taking a look at some alternative views of morality, starting with the basic question of whether morality has any independent existence at all.
Moral realism is the view that morality has an objective basis in reality, and so moral statements can be just as true or false as other propositions about the universe.
"This act of theft is wrong" can be just as true a statement as "This table consists of a large number of various atoms", or "the sum of the angles of a triangle on a Euclidean plane sum to 180 degrees", or "1+1=2". Furthermore, they are true in exactly the same sense.
Where the reality of morality stems from may or may not be specified. It may come from God, it may come from logic, it may be somehow as intrinsic to the universe as the laws of physics.
My main issue with moral realism is that I can see no basis from which one can conclude that moral statements have an objective truth or falsity, however much we might wish that to be the case. In "The Moral Landscape", Sam Harris proposes what he views as an objective basis for the reality of morality, an argument which I will discuss in a later post when I address his book directly.
Moral Nihilism / Skepticism / Anti-realism
This view holds that there is no such thing as intrinsic morality, or else that moral knowledge is impossible. Morality exists only as a social construction or evolutionary adaptation, and so there is no sense in which a moral value can be said to be true. Adherents of this view most famously include the likes of Macchiavelli and Nietzsche.
I do have a certain sympathy for this view, however I think it is important not to immediately jump from the non-existence of objective morality to the conclusion that all subjective morality is worthless, or that selfish amoral behaviour is somehow excused.
Wikipedia discusses two forms of moral nihilism: expressivism and error theory.
Expressivism maintains that when people say something is morally right or wrong, they are essentially expressing a preference, perhaps akin to an aesthetic preference. While I think this position most closely reflects my own, I would probably go farther and say they are expressing a belief or conviction which is at least felt to be more objective than a mere aesthetic preference.
Error theory maintains that when people engage in moral reasoning, they are simply making an error to which they are gentically and culturally disposed, and all such endeavours are ill-conceived. I do not hold with this view.
Instead, I believe that most of us have an innate sense of what is right and what is wrong, and furthermore that there is a great deal of consensus regarding this morality. Even though morality may not be universal or intrinsic to the universe, it nevertheless exists as an expression of what humanity as a whole and individually considers to be a description of appropriate and inappropriate modes of behaviour, desirable and undesirable outcomes. A moral judgement is not a mere preference, it is a belief. I as an individual believe that theft is wrong. My society, and most if not all other societies, believes that theft is wrong. When we say "theft is wrong", what we really mean is that "we hold theft to be wrong", in which case we are making a true statement. In this sense, morality clearly has some kind of existence, even if only in our minds.
So, which camp do you put yourself in? Moral realist or moral nihilist, or somewhere in between?