In Reply:--The concept that there are distinct single mechanisms for pain and for consciousness and that each could be influenced by multiple factors certainly is possible. However, it is difficult to understand that, because this hypothesis is simpler and more "parsimonious and testable," it is likely to be more valid than any multiple mechanisms concept. Biologic systems are complex. Like critical components of anesthesia machines, critical components for life often are backed up by "fail-safe" mechanisms.

Roger A. Johns, M.D., Professor of Anesthesiology, Department of Anesthesiology, University of Virginia Health Sciences Center, Box 238, Charlottesville, Virginia 22908.