David Selig is 41 and counting. Slightly above medium height, he has the lean figure of a bachelor accustomed to his own meager cooking, and his customary facial expression is a mild, puzzled frown. He blinks a lot. In his faded blue denim jacket, heavy-duty boots, and 1969-vintage striped bells he presents a superficially youthful appearance, at least from the neck down; but in fact he looks like some sort of refugee from an illicit research laboratory where the balding, furrowed heads of anguished middle-aged men are grafted to the reluctant bodies of adolescent boys. How did this happen to him? At what point did his face and scalp begin to grow old?

Aldous Huxley thought that evolution has designed our brains to serve as filters, screening out a lot of stuff that’s of no real value to us in our daily struggle for bread. Visions, mystical experiences, psi phenomena such as telepathic messages from other brains—all sorts of things along these lines would forever be flooding into us were it not for the action of what Huxley called, in a little book entitled Heaven and Hell, “the cerebral reducing valve.” Thank God for the cerebral reducing valve! If we hadn’t evolved it, we’d be distracted all the time by scenes of incredible beauty, by spiritual insights of overwhelming grandeur, and by searing, utterly honest mind-to-mind contact with our fellow human beings. Luckily, the workings of the valve protect us—most of us—from such things, and we are free to go about our daily lives, buying cheap and selling dear.