VISIONS. Usage of the term vision goes back to the thirteenth-century Italian theologian Thomas Aquinas, who first used the word to refer to a "supernatural" manifestation. It describes a religious experience that involves seeing and, frequently, the other senses as well. The quality of the experience suggests that the content of the perception is real, a direct, unmediated contact with a nonordinary aspect of reality that is external and independent of the perceiver. "[Vision] is very real," says Lame Deer, a medicine man of the Sioux nation. "It hits you sharp and clear like an electric shock. You are wide awake and, suddenly, there is a person standing next to you who you know can't be there at all" (Lame Deer, Seeker of Visions, New York, 1972, p. 65).

The explanation that visions are due to imaginings, pseudoperception, or errors of perception is an expression of the cultural difference between...