The Cotard delusion (also Cotard's syndrome and walking corpse syndrome) is a rare mental illness in which an afflicted person holds the delusion that they are dead, either figuratively or literally; yet said delusion of negation is not a symptom essential to the syndrome proper. Statistical analysis of a hundred-patient cohort indicates that the denial of self-existence is a symptom present in 69 percent of the cases of Cotard's syndrome; yet, paradoxically, 55 percent of the patients might present delusions of immortality.
In 1880, the neurologist Jules Cotard described the condition as Le délire des négations ("The Delirium of Negation"), a psychiatric syndrome of varied severity. A mild case is characterized by despair and self-loathing, and a severe case is characterized by intense delusions of negation and chronic psychiatric depression. The case of Mademoiselle X describes a woman who denied the existence of parts of her body and of her need to eat, and said that she was condemned to eternal damnation and therefore could not die a natural death. In the course of suffering "The Delirium of Negation", Mademoiselle X died of starvation.