McGinn presents an indirect, a priori argument for the conjunction of realism about the external world and realism about the mind. The argument is indirect because McGinn's principal targets are the alternative conjunctions that include some form of anti‐realism about either the external world (phenomenalism), the mind (behaviourism), or both: phenomenalism plus realism about the mind; realism about the external world plus behaviourism; phenomenalism plus behaviourism. McGinn argues that both of these anti‐realist positions are self‐refuting. ‘Behaviourism requires realism about material objects, since anti‐realism about material objects is inconsistent with behaviourism; but realism about material objects requires that behaviourism be false; so behaviourism is false.’ Moreover, ‘phenomenalism requires realism about the mind, since anti‐realism about the mind (behaviourism) is inconsistent with phenomenalism; but realism about the mind requires that phenomenalism is false, since it requires that behavioural statements not imply mental statements; so phenomenalism is false’. Therefore, McGinn concludes, we know a priori that ‘any argument (such as Dummett's) that purports to establish anti‐realism in either of these two areas has to be unsound’.