The functioning of actual patients who have had the two hemispheres of the brain severed is well documented. The procedure is one of cutting the corpus callosum or ‘thick-skinned body’, a procedure known as cerebral commisurotomy. It was formerly used as a way of controlling various disorders including epilepsy, and has been one of the main techniques for studying the localization of function within the brain, showing for example the dominance of the left hemisphere in linguistic behaviour. Such studies also provide data on the many different layers of functioning (and many different ways it can go wrong) underlying the familiar unities of conscious experience.

Philosophical attention to these matters is apt to simplify, legitimately or not, in order to consider the possibility of two persons cohabiting in one body, or that of one person, whose functions are subserved by each of the different hemispheres, being relocated in two different bodies. The relationship between these thought experiments and the actual facts is not close, for although the operation results in separate awareness in the right and left halves of the visual and auditory fields, away from experimental conditions subjects need to unify their experience, and except in cases of severe dysfunction do so as much as the rest of us do.