It is widely held, even among nonnaturalists, that the moral supervenes on the natural. This is to say that for any two metaphysically possible worlds w and w′, and for any entities x in w and y in w′, any isomorphism between x and y that preserves the natural properties preserves the moral properties. In this paper, I put forward a conceivability argument against moral supervenience, assuming non-naturalism. First, I argue that though utilitarianism may be true, and the trolley driver is permitted to kill the one to save the five, there is a conceivable scenario that is just like our world in all natural respects, yet at which deontology is true, and the trolly driver is not permitted to kill the one to save the five. I then argue that in the special case of morality, it is possible to infer from the conceivability of such a scenario to its possibility. It follows that supervenience is false.

Do psychopaths make moral judgments but lack motivation? Or are psychopaths’ judgments are not genuinely moral? Both sides of this debate seem to assume either externalist or internalist criteria for the presence of moral judgment. However, if moral judgment is a natural kind, we can arrive at a theory-neutral criterion for moral judgment. A leading naturalistic criterion suggests that psychopaths have an impaired capacity for moral judgment; the capacity is neither fully present nor fully absent. Psychopaths are therefore not counterexamples to internalism. Nonetheless, internalism is empirically problematic because it is unable to explain psychopaths’ moral deficits.

Current debates about ideal theory and idealization in modern moral and political philosophy do not typically scrutinize the form of reflection itself. This is an unfortunate oversight: assumptions about the form of reflection shape the positions defended in those debates. I argue that the appropriate form of reflection on the nature and justification of standards of justice and morality is the form of practical reason. I further argue that the form of practical reason cannot support many of the idealizations typically deployed in modern moral and political philosophy.