Obsessional prohibitions are extremely liable to displacement. They extend from one object to another along whatever paths the context may provide, and this new object then becomes, to use the apt expression of one of my women patients, 'impossible' - till at last the whole world lies under an embargo of 'impossibility'.

The Standard Edition of the Complete Psychological Works of Sigmund Freud: (1913-1914) Totem and taboo and other works. [1955 (ed. 1953)