Excerpt from The Man Who Wasn't There by Anil Ananthaswamy, plus links to reviews, author biography & more

Walk down the rue de l'École-de-Médecine in the Sixth Arrondissement in Paris, and you'll see a formidable colonnade. A striking example of French neoclassical architecture, the colonnade forms a portico for the Université René Descartes. Designed in the late eighteenth century by architect Jacques Gondouin, the façade, as the architect intended, demands attention and yet feels open and inviting.

I entered the building to visit the rare manuscript section of the Library of the School of Medicine, to look at a document on the life of Jules Cotard. The document is the text of a eulogy delivered by his friend and colleague Antoine Ritti in 1894, almost five years after Cotard's death. Cotard had been devotedly nursing his daughter, who was suffering from diphtheria, but then fell ill himself with the disease and died in 1889. Much of what we know of Cotard comes from Ritti's eulogy, a copy of which exists amid the pages of an old leather-bound volume, whose spine simply reads MÉLANGES BIOGRAPHIQUES—a mixture of biographies. I turned the pages to Ritti's eulogy. Handwritten on the first page was a note to the then head of the faculty of medicine of the university: "Hommage de profond respect," the note read. It was signed Ant. Ritti.

Cotard is best known for describing what are called nihilistic delusions, or délire des négations. But before he came up with that phrase, Cotard first talked of "delirium in a severely melancholic hypochondriac" at a meeting of the Société Médico-Psychologique on June 28, 1880, using as an example the case of a forty-three-year-old woman who claimed "she had 'no brain, nerves, chest, or entrails, and was just skin and bone,' that 'neither God or the devil existed,' and that she did not need food, for 'she was eternal and would live forever.' She had asked to be burned alive and had made various suicidal attempts."

Soon afterward, Cotard coined the phrase délire des négations, and after his death, other doctors named the syndrome after him. Over time, "Cotard's delusion" has come to refer to the most striking symptom of the syndrome—the belief that one is dead. However, the syndrome itself refers to a constellation of symptoms, and does not have to include the delusion of being dead or not existing. The other symptoms include the belief that various body parts or organs are missing or putrefying, feelings of guilt, feelings of being damned or condemned, and paradoxically, even feelings of immortality.

But it's the delusion that one does not exist that poses an interesting philosophical challenge. Until recently, the seventeenth-century French philosopher René Descartes's assertion Cogito ergo sum (I think, therefore I am) was the bedrock of Western philosophy. Descartes established a clear dualism of mind and body: the body was of the physical world, something that takes up space and exists in time, while the mind's essence was thought and it did not extend into space. For Descartes, cogito did not mean thinking as much as "clear and distinct intellectual perception, independent of the senses." An implication of Descartes's philosophy, according to philosopher Thomas Metzinger, was that "one cannot be wrong about the contents of one's own mind."

This Cartesian idea has been falsified in many disorders, including Alzheimer's, where patients are often unaware of their own condition. Cotard's syndrome is also a puzzle. Metzinger argues that we should be paying attention to what it feels like to be suffering from Cotard's—what philosophers call the phenomenology of a disorder. "Patients may explicitly state not only that they are dead, but also that they don't exist at all." While this seems logically impossible—an obviously alive individual claiming not to exist—it is part of the phenomenology of Cotard's.

Excerpted from The Man Who Wasn't There by Anil Ananthaswamy. Copyright © 2015 by Anil Ananthaswamy. Excerpted by permission of Dutton. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.