I'm a great admirer of what Oliver Sacks has done as a neurologist and much of his written work. The concept of those book is so important as it is critical that someone can explain to the non-specialist reader about the different sorts of visual, auditory, olfactory and tactile experiences that can be had. I hoped this would be the case with this book but the terminology undermines that. An hallucination suggests something that the recipient believes to be read. However, many examples given in the book did not fit that. If someone consciously takes a drug that makes them see or hear things not normally there, we can't assume that all would regard those objects, entities and sounds are real. Even people who have chronic schizophrenia routinely come to an awareness that voices they hear in their head are not real. They would only be an hallucination if believed to be real, despite evidence to the contrary. In these two examples given, with vision-enabling substances and psychosis, most theorists have moved on, with entheogen or psychedelic used as terms about the former (not hallucinogens) and, in the later, voices rather than auditory hallucination has been the terminology of psychiatrists and psychologists for decades now. While the author at least did go through the cultural history of visionary experiences, the use of the title term actually can reinforce ignorance in this very important area of human experience