The first and foremost concrete fact which every one will affirm to belong to his inner experience is the fact that consciousness of some sort goes on. I -William James, 1893 We are witnessing today a mounting interest among behavioral and biological scientists in problems long recognized as central to our understanding of human nature, yet until recently considered out of the bounds of scientific psychology and physiology. Sometimes thrown into the heading of "altered states of consciousness," this growing research bears directly upon such time-honored questions as the nature of conscious...