 So on a more charitable reading, the incompatibilist’s reductio does not target an externalism based on a causal theory. It is rather directed at a view based solely on the Twin Earth thought experiments, given that the experiments are more naturally seen as apriori. (But see Baker 2007.) For the remainder of section 2, ‘externalism’ and ‘EXT’ shall refer exclusively to this kind of Twin Earth based externalism.