How is the author qualified to write a book about hallucinations?

Oliver Sacks is the author of the novel, and a well known author and scientist. Sacks has worked in many fields that qualify him as an expert on the topic of hallucinations including his work with encephalitis patients, migraine patients, those with phantom limbs, and elderly patients with Charles Bonnet Syndrome, Parkinson's, and other diseases which cause frequent hallucinations. Additionally, Sacks himself admits to experimenting heavily in hallucinogenic drugs over the course of his younger years, which has allowed him a rare personal insight into how these hallucinations manifest. Still further, Sacks has researched hallucinations heavily, and supplements his own first hand experiences with both historical and modern accounts of the diseases and symptoms he writes about. His motivations for writing the book are clearly not only to alert the public to the commonality of hallucinations for a variety of reasons, but also to give possible solutions to many modern day mysteries, including ghosts, God, myths, and cultural icons. Because his writing style is both intellectual and humorous, his novel appeals to a wide audience, and his work is undeniably one of the most cumulative discussions of hallucinations outside of mental illness.