Sacks does not, I think, adequately distinguish two types of hallucination. I will call them doxastic and phenomenal. In a phenomenal hallucination of A, one has sensory experience as of A. In a doxastic hallucination, one thinks one has sensory experience as of A. The two can come apart.
Consider this description, from page 99 of Hallucinations (and attributed to Daniel Breslaw via David Ebin's book The Drug Experience).
The heavens above me, a night sky spangled with eyes of flame, dissolve into the most overpowering array of colors I have ever seen or imagined; many of the colors are entirely new -- areas of the spectrum which I seem to have hitherto overlooked. The colors do not stand still, but move and flow in every direction; my field of vision is a mosaic of unbelievable complexity. To reproduce an instant of it would involve years of labor, that is, if one were able to reproduce colors of equivalent brilliance and intensity.Here are two ways in which you might come to believe the above about your experience: (1.) You might actually have visual experiences of the sort described, including of colors entirely new and previously unimagined and of a complexity that would require years of labor to describe. Or (2.) you might shortcut all that and simply arrive straightaway at the belief that you are undergoing or have undergone such an experience -- perhaps with the aid of some unusual visual experiences, but not really of the novelty and complexity described. If the former, you have phenomenally hallucinated wholly novel colors. If the latter, you have merely doxastically hallucinated them.
The difference seems important -- crucial even, if we are going to understand the boundaries of experience as revealed by hallucination. And yet phenomenal vs. merely doxastic hallucinations might be hard to distinguish on the basis of verbal report alone, and almost by definition subjects will be apt to confuse the two. I can recall no point in the book where Sacks displays sensitivity to this issue.
Once I was attuned to it, the issue nagged at me again and again in reading:
Time was immensely distended. The elevator descended, "passing a floor every hundred years" (p. 100).The possibility of merely doxastic hallucination might arise especially acutely when subjects report highly unusual, almost inconceivable, experiences or incredible detail beyond normal perception and imagery; but of course the possibility is present in more mundane hallucination reports too.
Then my whole life flashed in my mind from birth to the present, with every detail that ever happened, every feeling and thought, visual and emotional was there in an instant (p. 102).
I have had musical hallucinations (when taking chloral hydrate as a sleeping aid) which were continuations of dream music into the waking state -- once with a Mozart quintet. My normal musical memory and imagery is not that strong -- I am quite incapable of hearing every instrument in a quintet, let alone an orchestra -- so the experience of hearing the Mozart, hearing every instrument, was a startling (and beautiful) one (p. 213).
(A fan of Dennett might suggest that there's no difference between the phenomenal and doxastic hallucinations; but I don't know what Dennett himself would say -- probably something more complex than that.)