In the 1930s, several members of the Vienna Circle set out to incorporate psychology in the unity of science, by showing that all cognitively significant sentences of psychology can be translated into the language of physics. This epistemological analysis of psychology has become known as logical behaviourism. Carnap was the first logical empiricist to expound this programme in considerable detail. Relying on his particular notion of protocol languages, Carnap develops a view on the philosophy of psychology that not only is thoroughly physicalistic, but also shows due appreciation to 'introspection' as a purely subjective, but reliable way to verify sentences about one's own mind. Second, contrary to the received view on logical behaviouris...