Gilbert Harman begins his essay “Is There a Single True Morality?” by telling us:

Having said this, however, he immediately admits that this opinion is not shared by many of his philosophical colleagues, and he goes on to explore the issue that divides them. Harman’s hypothesis is that they have different attitudes towards naturalism. Naturalism, he tells us, decisively favors the view that basic moral demands apply only to some. He therefore spends the bulk of his paper trying to make out that connection. This is an extremely important conclusion, if it is correct, so my aim in what follows is to consider the arguments Harman provides to support his hypothesis. To anticipate, I will argue that the view that moral demands apply only to some gains no support from naturalism, and I attempt to identify where the real source of support for that view lies.