The Duplication Argument Defeated

Abstract

Suppose I am a nuclear engineer in the twenty-second century and, following accidental exposure, am dying of radiation sickness though this has not yet affected my central nervous system. My monozygotic twin brother, raised separately by foster parents in South America, is summoned to my bedside but becomes involved in a car crash just outside the hospital, sustaining a concussion that provokes brain death; his body is kept alive on a respirator in the room next to mine. With my permission and that of my brother's foster parents, my brain is transplanted into and hooked up with his body. The operation is successful: my brain takes control of my brother's body. Who is the survivor? Of course the postoperative survivor S looks very much as I did prior to my wasting illness, given that we were identical twins. But upon closer inspection S looks even more like my brother. Yet S would have none of my brother's memories of life in South America, would speak no Spanish and find my brother's faithful mistress a total stranger. On the other hand, he would love my wife and look upon my children as his own. In fact all S's persistent psychological characteristics, from skills down to sports preferences and tastes in food, would match those I had preoperatively. In a word, S would be psychologically continuous not with my brother but with me. Since no human brain has been transplanted in the above manner, how can I be sure this would be the outcome? Here I introduce two empirical assertions. The first is that it is the human brain, and not the body as a whole, which is the biological substrate of all persistent and individuating psychological characteristics, including the memories one has, that are displayed in human behaviour. The evidence for this assertion would be I50 years of clinical psychopathology, detailing the effects on behaviour of brain damage and disease, plus the wide variety of reliable neuroscientific studies done more recently with normal as well as brain-damaged people. My second assertion, following hard on the first, is that whether human beings know this or not, it is the spatiotemporal continuity of a particular living human brain which accounts for the development and persistence of those individuating psychological characteristics displayed in behaviour. The notion that one could exchange one's brain, the way one can exchange one's heart, for another and remain the same psychological entity may not be contradictory, but it is scientific nonsense. Thus it appears that psychological continuity, rooted in the numerical identity through time of a particular living brain, would be decisive in judging that S is neither my brother nor some brand new person (who inexplicably loves my wife, of all people), but is me. This finding, when combined with the medical records, could certainly 582