Naturalism and normativity

Synthetic naturalism is a form of moral realism which holds that we can discover a posteriori that moral properties exist and are natural properties. On this view moral discourse earns the right to be construed realistically because it meets the conditions that license realism about any discourse, that properties it represents as existing pull their weight in empirical explanations of our observations of the world. I argue that naturalism is an inadequate metaphysics of moral value, because parallel arguments to those used by the naturalist to establish the reality of 'moral' properties and their normativity for persons of sympathetic temperament can be constructed, which would equally demonstrate the reality of normatively antagonistic value properties, and their normativity for differently psychologically constituted agents. Since moral discourse implicitly denies that there are such diverse and competing normative truths the strategy fails to establish moral realism.