Abstract: That knowing is a mental state is inconsistent with internalism, the claim that whether one is in a given mental state depends only on what is going on inside one's head, for the truth of what one knows may involve the external environment. More familiar kinds of externalism in the philosophy of mind have concerned the content of a mental state; the present view extends externalism to the propositional attitudes to those contents. Attempts to extract belief or rational belief as the internalist, purely mental core of knowledge are refuted. The causal efficacy of knowing is upheld by a consideration of cases in which our best causal explanation of an extended action appeals to knowing rather than believing.

Keywords: action, belief, causal, explanation, externalism, internalism, mental state, propositional attitude,