What if you woke up tomorrow and you believed you were dead? Like really believed it? That’s Cotard’s Syndrome.
It’s almost a Camus-level existential mindwarp. Believing that you do not, in fact, exist. James Byrne writes about its history and recent work to uncover its cause at Scientific American:
The first described patient was presented in a lecture in Paris in 1880 by Jules Cotard as Mademoiselle X who presented with significant self-loathing manifested as a denial of the existence of god or the devil and several parts of her body. As she believed herself to be eternally damned and incapable of dying a natural death and so no longer needed to eat. She later died of starvation in what one must assume came as a shock to her at least.