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

Ralph Waldo Emerson is thought to have suffered from Alzheimer's. He also wrote eloquently about memory and its role in making us who we are. But Emerson was curiously indifferent about his own dementia. It's one of the characteristics of Alzheimer's disease that sufferers are sometimes unaware of their own condition. Alzheimer's was the unmaking of his identity, including identifying himself as diseased.

The next chapter examines Alzheimer's and its role in the undoing of a person, while asking: is some essence of selfhood—despite a ravaged brain in the late stages of disease—preserved in the body? The celebrated American composer Aaron Copland (1900–1990) also suffered from Alzheimer's disease. At times he wouldn't know where he was, but he could still conduct his signature orchestral suite Appalachian Spring. Who or what swung the conductor's baton?

Body integrity identity disorder—a curious condition in which people feel that some part of their body, usually limbs, is not their own, often leading them to the horrific act of severing the body part—gives us a glimpse into how the brain constructs a sense of one's own body, the bodily self.

Schizophrenia can fragment a person—and part of this fragmentation is due to a compromised sense of agency, the feeling we all have that we are the agents of our actions. What if this feeling—a crucial aspect of the self—goes awry? Could it lead to psychosis?

Then there is depersonalization disorder, which robs the self of its emotional substrate, making us strangers to ourselves, thus highlighting the role of emotions and feelings in creating the self.

Autism sheds light on the developing self. Children with autism are usually unable to instinctively "read" others' minds, which then leads to problems relating socially to others, but is this ability also tied to reading one's own mind and hence self-awareness? There's tantalizing new work suggesting that the roots of this impairment lie in an autistic brain's inability to make sense of the body and its interactions with the environment, leading first to an uncertain bodily self and then to behavioral problems.

Out-of-body experiences and the more complex doppelgänger effect (in which people perceive and interact with a duplicate of their own body) reveal that even the most basic things we take for granted—being grounded in a body, identifying with it, and viewing the world from behind our eyes—can be disrupted, thus giving us a glimpse of the components necessary for a low-level self that potentially precedes all else.

Ecstatic epilepsy begets a condition that borders on the mystical, when we are truly here and now, fully aware of our own being, yet paradoxically bereft of boundaries, leading to a feeling of transcendental oneness. Is this condition bringing us closer to the essence of the self—a self that maybe endures for just moments and is at the heart of the debate about whether there is or there isn't a self?

We conclude with a journey to Sarnath, India, where the Buddha, nearly 2,500 years ago, gave his first sermon. Buddhist ideas of no-self seem to resonate with what some modern philosophers are saying about the self—that it's illusory. But is it really? Does empirical evidence support the idea that the self is a made-up entity? Insights gleaned from the maladies of the self will help us make sense of age-old questions and maybe even ask a few of our own.

While visiting David Cohen in Paris, I asked him about May, his fifteen-year-old Cotard's syndrome patient. "Who is it that is saying she doesn't exist?"

"This is the mystery of psychiatry," Cohen said. "We always say that there is something . . . that can still relate to the real world, even in the most crazy state."

In Liège, Steven Laureys's PhD student Athena Demertzi, who helped Laureys scan and study Graham, told me something about Graham that also reminded me that despite his delusion of being brain dead, there was an essence that remained. Graham had just come out of the scanner when Demertzi asked him, "Are you OK?"

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.