Looking down on Athens from the Acropolis, a first-time visitor observed, ‘So all this really does exist, just as we learnt at school!’ As he noted later, ‘the person who gave expression to the remark was divided … from another person who took cognizance of the remark’. The first person was surprised to see something whose reality had seemed doubtful, the second astonished that the reality of the Athenian landscape could be in doubt. Both people were named Sigmund Freud.

Freud’s fleeting experience of estrangement is a well-known example of what came to be called derealisation. It frequently co-occurs with depersonalisation, a feeling of disconnection from self. These phenomena share a sense of alienation and unreality and fall on a spectrum extending from momentary wooziness to chronic mental illness. Once understood as expressions of hysteria, depersonalisation and derealisation later became recognised as among a group of dissociative conditions that involve alterations of consciousness, selfhood, and memory.