The stream of consciousness—that flow of perceptions, purposeful thoughts, fragmentary images, distant recollections, bodily sensations, emotions, plans, wishes, and impossible fantasies—is our experience of life, our own personal life, from its beginning to its end. As scientists, we may approach the subject for the joy of discovering how it works. As clinicians, therapists, and social engineers, we may study it in order to reduce human suffering. But simply as people, we are drawn to it precisely because it is that portion of our being at once most familiar and most mysterious.