Metaethical questions are typically held to be a priori , and therefore impervious to empirical evidence. Here I examine the metaethical claim that motive-internalism about belief (or belief-internalism), the position that moral beliefs are intrinsically motivating, is true. I argue that belief-internalists are faced with a dilemma. Either their formulation of internalism is so weak that it fails to be philosophically interesting, or it is a substantive claim but can be shown to be empirically false. I then provide evidence for the falsity of substantive belief-internalism. I describe a group of brain-damaged patients who sustain impairment in their moral sensibility: although they have normal moral beliefs and make moral judgments, they are not inclined to act in accordance with those beliefs and judgments. Thus, I argue that they are walking counterexamples to the substantive internalist claim. In addition to constraining our conception of moral reasoning, this argument stands as an example of how empirical evidence can be relevantly brought to bear on a philosophical question typically viewed to be a priori .