The author takes as his starting point the view that mental health requires the existence of a satisfactory relationship between internal (psychic) and external reality. He discusses the nature of the ‘membrane’ that exists between these two realities and the qualities required for it to function adequately. He explores the possible developmental origins of this ‘membrane’, and the different ways in which its establishment may miscarry. He illustrates one of the ways in which this process may go wrong when the ‘membrane’ has become excessively reinforced. He illustrates this with clinical material from a schizoid, adult patient who had taken refuge in external reality and with material from a child patient with autistic spectrum disorder. Links are made to Freud's views on the establishment of the Reality Principle, the process of splitting and the importance of the mechanism of disavowal. There is some speculation as to the possible aetiology of the pathology in the two cases described, and the developmental importance of acquiring a valid sense of potency.