In a book I'm reading for an Ethics class, Joyce Milton describes how nineteenth century philosopher William James was "persuaded that our normal waking consciousness is just one of many 'potential' forms of consciousness that exist somewhere beyond our reach, separated from the waking mind by the filmiest of screens." He came to this conviction after he had experimented with mind altering drugs in attempt to have an experience of some "mystical significance."
Milton comments on James' reflections on his encounter: "Unfortunately, [the essence of this experience] was private, nonverbal and fleeting."
The problem with the mystical experience was that it lacked any authority to communicate itself to others.
The Road to Malpsychia: humanistic psychology and our discontents, p. 69.