Compatibilism combines an externalist view of mental content with a doctrine of privileged self‐knowledge. The essay presents a reductio of compatibilism by arguing that if compatibilism were true, we would be in a position to know certain facts about the world a priori, facts that no one can reasonably believe are knowable a priori. Whether this should be taken to cast doubt on externalism or privileged self‐knowledge is not discussed. Consideration is given to the ’empty case’—the case in which a (...) thinker expresses what he takes to be a genuine thought concerning a natural kind but where there is in fact no relevant natural kind, so that the putative natural kind term fails to refer. It is argued that, in such a case, on an externalist conception of natural kind terms, the thinker is deluded in taking himself to be expressing a complete thought, and that such delusions are not compatible with privileged self‐knowledge. (shrink)