 Distinctions between internalism and externalism arise in many areas in philosophy—philosophy of mind, philosophy of language, epistemology, and ethics. All of these distinctions can be characterized by means of supervenience theses. For example, an internalist about mental content accepts, and an externalist denies, that what a mental state is about supervenes upon neurophysiological properties, or “what’s in the head.” The dispute is about whether two thinkers can differ in the contents of their mental states without also differing in some neurophysical respect. Similarly, an internalist about epistemic justification accepts, and an externalist denies, that whether a belief is justified supervenes upon the mental properties of the believer. The dispute is about whether two believers can differ in whether their beliefs are justified without also differing in some mental respect. And an internalist about moral motivation accepts, and an externalist denies, that moral motivation supervenes upon moral judgment. The dispute is about whether two (rational) people can differ in how they are motivated to act without also differing in their judgments about which of those actions they ought to perform.