Thought experiments: Split brain

Since David Wiggins and his article of 1967, the reflection on personal identity, one of the most difficult and disturbing in the philosophy of mind, has often relied on the “argument from fission” to challenge our most ingrained beliefs about it, such as the belief that our consciousness is “one”. This argument starts from a fiction which consists first of imagining that our brain is severed at the level of the corpus callosum separating our two cerebral hemispheres, then that each of them is transplanted into the empty cranial box of two other individuals. A double question then follows for us: “Do I survive the operation? and “If so, where am I?” “. It is on the basis of this thought experiment that the philosopher Derek Parfit justifies, against common sense, the idea that it is not necessary to preserve one’s individuality to survive. Medical techniques have shown that some individuals manage to survive with only one cerebral hemisphere. The successful transplantation of at least one of my two cerebral hemispheres must therefore be enough for my survival. The “fission” operation, if successful for both hemispheres, therefore implies that I survive through my two avatars, although they each constitute a distinct person. In order to eliminate this paradox, the philosopher Roland Puccetti went so far as to assert that even the normal human being, with an intact brain, actually harbored two people.

Derek Parfit has amply developed a whole science fiction in order to undo our usual conception of personal identity, a conception which notably includes the belief in the uniqueness of consciousness and in the individuality of the “me”. One of his famous fictions is that of the teleporter, an extremely sophisticated machine that allows us to travel in an instant to a foreign planet. The simplest case of this fiction is the following: the device which scans our body destroys it at the same time as it transmits all the characteristics of this body (brain included) to a receiving machine on Mars, which takes care of reconstituting us in organic material. Insofar as our consciousness depends directly on our cerebral composition, our Martian double has the memory of our terrestrial entry into the transport device as well as of all our life preceding this moment. Although he is not in a relationship of physical continuity with us, since the elements of the old body have all been replaced, this double is in a relationship of maximum psychological continuity with us and thinks that he is the same person as the one who entered the device. For Parfit, this thought experiment shows that survival does not necessarily imply spatio-temporal continuity, or even physical identity, but only psychological continuity.

(Includes texts from Wikipedia translated and adapted by Nicolae Sfetcu)