Where does our sense of right and wrong come from? Most people think it is a gift from God, who revealed His laws and elevates us with His love. A smaller number think that we figure the rules out for ourselves, using our capacity to reason and choosing a philosophical system to live by.
Moral naturalists, on the other hand, believe that we have moral sentiments that have emerged from a long history of relationships. To learn about morality, you don’t rely upon revelation or metaphysics; you observe people as they live. . . .
Read the rest here: http://www.nytimes.com/2010/07/23/opinion/23brooks.html?_r=2.