The intuition that led Terence and me to write The Invisible Landscape was that the key to the brain/mind problem and perhaps many other questions of considerable ontological significance for our species lies in a thorough understanding of the psychedelic experience, from its molecular mechanisms to its historical and evolutionary implications. That original intuition has remained a valid working hypothesis to this day, and in fact has been the stimulus, at least in our lives, for most of our subsequent intellectual development. In that time we have partially revised our interpretations of the precipitating causes and consequences of our experiences, have acquired new information, and have examined alternative hypotheses ranging from the pharmacological to the mythical; but always driving the searching and questioning has been the desire to understand the nature of the psychedelic experience. We are, if anything, less assured of the success of our quest today than when this book was written, but remain convinced that any model of the brain/mind that does not reconcile the observations of neurobiology with the fact of the psychedelic state, as it is experienced, is doomed to remain scientifically incomplete and philosophically unsatisfying.