This ultimate end of everything, this diametric opposite of all, is nothing in itself. And yet we think of nothing as something. Clearly, “nothing” is that conceptual entity that shows most clearly the illusory status of all conceptual entities. To use the verb “exist” about it is a misuse of language. To refer to nothing as “it,” “this,” “that,” or whatever, without quotes, is a mistake, as only the concept can be referred to. “Nothing” is categorically unimaginable, so it is incorrect to picture nothing as a destination. It is wrongly imagined to be a ground state giving off no reading, which appears as an EEG flatline. It is incoherently imagined to be an anesthesia following a loss of all sensation. It is wrongly conceived to be a place of quiet within which consciousness can slip. It cannot be entered.
When we do imagine nothing in the above ways, we are actually doing nothing more than comforting ourselves. We are building our image of the state of nothingness on our habitual sense of deep sleep. We lose concerns, worries, sensations, and finally consciousness itself as we fall asleep. We “fall” asleep, having a sense of relaxing into a ground state. We may have a last moment of awareness of slipping away, we may have a first moment of waking up complete with a memory of consciousness out of a dream. But we do not have any direct experience of being unconscious, so we cannot remember having been unconscious. Thus our sense of having rested in a state of nothingness of consciousness is an inference based on the last and first moments of threshold awareness.
from The Tibetan Book of the Dead
translation by Robert Thurman