Our normal waking consciousness is but one special type of consciousness. Whilst all about it, parted from it by the filmiest of screens, there lie potential forms of consciousness entirely different. No account of the universe in its totality can be final that leaves these disregarded. How to regard them is the question – for they are so discontinuous with ordinary consciousness.
William James, one of psychology’s founding fathers, ruminating on his experiences with hallucinogenics. Quoted by Prof David Nutt in The Psychologist.