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

I left the library, and stepped back out onto the rue de l'École-de-Médecine, and turned around to take another look at the name "Université René Descartes" etched into the stone above the colonnade. There was something intriguing about researching Jules Cotard in a university named after Descartes. What does Cotard's eponymous delusion say about Cartesian ideas? Is the Cotard's syndrome patient saying, "I think, therefore I'm not"?

"Who is the I that knows the bodily me, who has an image of myself and a sense of identity over time, who knows that I have propriate strivings? I know all these things, and what is more, I know that I know them. But who is it who has this perspectival grasp?"

Who, indeed. The American psychologist Gordon Allport's lyrical musings above capture the central conundrum of being human. We instinctively and intimately know what he's referring to. It is there when we wake up and slips away when we fall asleep, maybe to reappear in our dreams. It is that feeling we have of being anchored in a body we own and control, and from within which we perceive the world. It is the feeling of personal identity that stretches across time, from our first memories to some imagined future. It is all of these tied into a coherent whole. It is our sense of self. Yet, despite this personal intimacy we have with ourselves, elucidating the nature of the self remains our greatest challenge.

All through recorded history, it is clear that humans have been fascinated and confounded by the self. Pausanias, a Greek traveler during Roman rule, wrote about the maxims inscribed at the fore-temple at Delphi by seven wise sages. One maxim said, "Know thyself." The Kena Upanishad, among the most analytical and metaphysical of Hindu scriptures, begins with these words: "By whom commanded and directed does the mind go towards its objects? . . . At whose will do men utter speech? What power directs the eye and the ear?"

Saint Augustine said this of the notion of time, but he might as well have been speaking about the self: "If no one asks of me, I know; if I wish to explain to one who asks, I know not."

And so it is that from the Buddha to the modern neuroscientist and philosopher, humans have pondered the nature of the self. Is it real or an illusion? Is the self in the brain, and if so, where in the brain is it? Neuroscience is telling us that our sense of self is an outcome of complex interactions between brain and body, of neural processes that update the self moment by moment, the moments strung together to give us a seamless feeling of personhood. We often hear of how the self is an illusion, that it is nature's most sophisticated sleight of hand. But all this talk of tricks and illusions obfuscates a basic truth: remove the self and there is no "I" on whom a trick is being played, no one who is the subject of an illusion.

From the Université Rene Descartes, it's a thirty-minute walk down rue des Écoles, past the national museum of natural history, to reach the Pitié-Salpêtrière Hospital, where Jules Cotard started his medical career as an intern in 1864. I went there to see David Cohen, the head of the hospital's infant and adolescent psychiatry unit.

Over the course of his medical residency and practice, Cohen has seen a few handfuls of patients who have suffered from Cotard's syndrome. Given the rarity of this disorder, this relatively large sample has given Cohen an intimate look at Cotard's. We talked of one particular patient, fifteen-year-old May—one of the youngest recorded cases of Cotard's. Cohen treated her and had extensive discussions with her after she recovered, enabling him to link her delusions with her personal history. He got a peek into how the self, even in a delusional state like Cotard's, is influenced by one's personal narrative and even dominant cultural norms.

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.