In his book entitled 'Wahn' (delusion), Binswanger develops a new line of thinking which is important for the neurosciences today. Unlike Jaspers, Binswanger believes that in a delusion normal perceptions do not acquire an abnormal significance. However, a delusion can develop as a result of a disturbance in sensations, perceptions and structure of consciousness. Binswanger was inspired to express these views after studying the philosophy of Heidegger, Aristotle, Kant and Husserl. In Binswanger's opinion, spontaneous mental activity is disturbed in delusion and perceptions are freezed. This can create a situation where the subject is dominated by mental images that can overpower and even destroy his own self-image. As a result the subject no longer has access to the human horizon which he would normally share with others. Consequently, the subject 'is convinced' that he is in the clutches of an alien power. On the basis of a case study, we investigate how a delusion arises and show how by analysing a patient's perceptions and sensations psychiatrists can help a patient to 'repossess his own mind'.