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Metaphysics

Could Two People Inhabit One Body?

Nicola Webber asks you (and you).

When we think of a person, we usually think of someone with a brain and a single unified consciousness. This view has come under fire from some philosophers who argue it’s possible for more than one person to inhabit a body – a possibility hinted at through ‘split brain’ patients, who have had the cerebral commissures (nerve fibres) linking their brain hemispheres cut. Different consciousnesses in each brain hemisphere would mean that two people can cohabit a brain. I will be considering arguments from Roland Puccetti, Eugene Mills and Derek Parfit, all leading to the conclusion that not only is it possible for two people to be in the same place at the same time, but that it’s always potentially so. I’ll be arguing from the standpoint that what matters for personal identity is psychological continuity and connectedness, not bodily continuity.

Puccetti argues that in patients where the nerve fibres linking the brain hemispheres have been severed in order to prevent interhemispheric epileptic seizures, the result is a doubling of consciousness, with two autonomous mental processes at work in one body. The left side of the brain controls and so displays mathematics and linguistics, while the right side has only a childlike linguistic ability, and controls artistic creativity. In a minority of cases the two hemispheres have little difference in ability, so if their cerebral commissures were severed the result would be two consciousnesses with equal abilities. This would be most likely to occur if brain bisection were done at infancy.

Puccetti conducts a thought experiment where an infant has her brain bisected, and from then on leads two different lives. The girl spends Monday to Thursday being called Amanda and living in Paris with her French family, and Friday to Sunday as Lucy, a girl from Birmingham. No part of Amanda’s life overlaps with Lucy’s. They regard a different house as home, different people as friends, and different languages as mother tongues. To explain the time lapses, Lucy and Amanda are told that they have short term amnesia. In this situation it would be illogical to point at the body of Amanda/Lucy and say “there’s one person”, for the truth is that they’re two people sharing the same body.

What if Amanda realised she shared a body with Lucy? Even if they realised the duplicity of their lives, it would be irrelevant for the two-person conclusion.

Parfit has a similar take to Puccetti on patients who have had their brain hemispheres separated, and so I’ll use one of his examples to support this conclusion about Amanda and Lucy. Parfit considers the implications of somebody being able to choose to separate their brain hemispheres by raising an eyebrow. In times of high pressure a person could raise their eyebrow and split into two consciousnesses, each one being able to write down what they were thinking. Then the brain could unite again. But during the split, each hemisphere would be subject to different thought processes. Thus this example allows for dual identity with a degree of control. A man’s body could lead two different lives with two different wives, and if caught cheating, could argue that his behaviour cannot be classified as cheating since with each woman a different hemisphere of his brain was being used, and he was therefore a different person!

Following Parfit, one can also imagine Frank and his wife Jane being in a car accident. Frank receives damage to the right hemisphere of his brain, while Jane receives damage to the left hemisphere, as well as her body being destroyed. While Jane is on life-support, Frank is approached by scientists telling him that they would like to test a new procedure for removing half a brain from one person and putting it into another person. Frank is told that the only way for Jane to survive is for her remaining brain to share his body, and so he agrees.

When Frank’s body wakes up, whose mind will it have? Frank’s, Jane’s, both, or neither? I hold that a bodily criterion is not enough for personal identity, and so one cannot say purely because of bodily continuity that the person who wakes up is Frank. My conclusion is that both Frank and Jane would survive, in Frank’s body, as there would be enough psychological connectedness with both Frank and Jane in the new brain/mind for us to say they cohabit the same body at the same time.

Eugene Mills considers bodily fission. Here surgical division results in two bodies, both being psychologically continuous with one original. Mills argues that the logical conclusion in this case is to say that the two people previously existed in one body.

A-body accidentally ran over a man while driving. Wanting to avoid prosecution, he drove away. Unfortunately, someone photographed the accident, giving pictorial evidence of one person in A-body’s car. However, as soon as A-body got home he went into a fission machine. What came out of the machine was B-body and C-body, each with one of the hemispheres of the brain that was previously in A-body’s body. B-body and C-body are two bodies, yet the photograph clearly shows one body in the car. Does this mean that the two new bodies are not responsible for the crime? If bodily identity is what matters when considering what constitutes a person, neither B-body nor C-body could be held responsible. I dispute this judgement since B-Body and C-body are both psychologically connected with A-body, and both could say “I remember the accident.”

I think the only plausible conclusion to draw is that previously A-body was cohabited. Although B-body and C-body now have distinct thoughts, pre-fission they were ‘psychological twins’. It would be counterintuitive to point to both B-body and C-body and say “there goes A-body!” One must acknowledge that they’re two separate people, and so two separate people previously cohabited A-body. Therefore I believe that it is possible for two people to cohabit the same body, and thus that two people can be in the same place at the same time.

Nicola Webber studied History and Philosophy and is now the Administrator of the Jewish Military Museum, Hendon, London.