In a seminar on Metaethics (h/t John Brunero) , I encountered an argument against moral facts that I hadn’t heard before. Here is a brief sketch:
(1) We’re justified in believing in some fact only if it plays a role in the explanation of our observations and other non-moral facts.
(2) Moral facts don’t play this role.
(3) We are not justified in believing moral facts.
In order to motivate (1), we can appeal to some flavors of naturalism. Many will argue that a completed science will account for (or give an explanatory account of) everything that exists. That is, a completed science will explain all physical phenomena. We’re justified in believing in electrons, in neurons, and in germs, insofar as they explain our observations of the natural world.