The American psychologist William James published The Varieties of Religious Experience near the turn of the last century, shortly after Freud's Interpretation of Dreams. Unlike Freud, who dismissed religious experience as neurotic wish fulfillment, James took his subject seriously. James' work was based on 200 personal narratives, but the impetus was clearly his own early experiments with nitrous oxide, which in addition to its anesthetic properties also had certain mind-altering effects. Although James was not a mystic, he believed that the state of consciousness he experienced under the influence of nitrous oxide was analogous. He reported that "the keynote of the experience is the tremendously exciting sense of an intense metaphysical illumination."
Even before Freud's views took hold, the scientific community tended to regard mystical experience as a form of self-induced delirium. But James argued that it was a psychological phenomenon that must be reckoned with on its own terms. In The Varieties of Religious Experience, he wrote, "Our normal waking consciousness, rational consciousness as we call it, 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."
Strip away the doctrinal and ritual trappings of most religions, and you will find some core of personal experience in which the curtain of normal waking consciousness has been parted. "At its heart, most theology, like most fiction, is essentially autobiography," wrote Frederick Buechner. The religion of the ancient Hebrews was forged in the personal experience of a shepherd tending his flocks on a mountain slope in the Sinai wilderness. The shepherd, a fugitive Egyptian prince named Moses, saw a bush that burned but was not burnt. He turned aside to see, and on this turning turned the whole history of Western religion. The account of this experience that has passed down to us does not mention other witnesses, so we must assume it is based solely on Moses' own testimony.
Moses reported that a voice spoke to him out of the bush and told him to remove his shoes "for the place on which you are standing is holy ground." The essence of one's spiritual journey, if we may call it that, is to find this holy ground. William James' contribution to our understanding of the process is to make it clear that this quest involves shifting the ground of consciousness. Far from seeing things that are not there, mystical consciousness is learning to see what is. The search for holy ground ends only when we discover that we are already standing on it.
Dimitri Tymoczko, "The Nitrous Oxide Philosopher: Do Drugs Make Religious Experience Possible?", Atlantic Monthly (May 1996)