I explained the unity of consciousness in terms of something extraneous to consciousness, a material co-consciousness relation. Such an account is not available to a sceptic about the external world who, I will argue (taking Hume as a paradigm case), cannot give any account of mental unity. Hume tries to say why we consider a diverse collection of perceptions to be a single thing, but on second thoughts confesses that he cannot find a satisfactory theory. This despair was well founded. Given the ontology of his most sceptical moments, when he believed that all that existed was his ideas and impressions, there was no other conclusion that Hume could arrive at. The result is scepticism about the external world and an inadequate conception of the mind. The best-explanation response to scepticism can also yield a better account of the mind which will ground my theory of mental unity.