¹ Before Commissurotomy surgery, a considerable amount was discovered about hemispheric specialization from the study of brain lesions: see Luria, A. R., The Working Brain (London, 1973).Google Scholar

² As far as I know, no left-handers have undergone commissurotomy surgery; they may prove to have speech partially represented in both hemispheres. See Levy, J., ‘Possible Basis for the Evolution of Lateral Specialization of the Human Brain’, Nature CCIV (1969), 614CrossRefGoogle Scholar. In any case, the degree of cerebral dominance varies considerably from person to person.

³ Seen by the author in Princeton, 1971.

⁴ Nagel, T., ‘Brain Bisection and the Unity of Consciousness’, Synthese XXII (1971) 396–413CrossRefGoogle Scholar; reprinted in Glover, J. (ed.), The Philosophy of Mind (Oxford University Press, 1976), 111–125.Google Scholar

⁵ Puccetti, R., ‘Brain Bisection and Personal Identity’, British Journal for the Philosophy of Science XXIV (1973), 339–355.CrossRefGoogle Scholar

⁶ Op. cit., 411.Google Scholar

⁷ Op. Cit., 351.Google Scholar

⁸ I owe this point entirely to David Wiggins.

⁹ A hemispherectomy is the surgical removal of all, or almost all, of one hemisphere of the brain.

¹⁰ See Squires, R., ‘On One's Mind’ philosophical Quarterly XX (1971), 347–356Google Scholar. For the moment I shall not quibble with the assumption that a ‘unity of consciousness’ either is, or is a prerequisite for, a mind. I argue against this in section V.

¹¹ Proust, M., A la Recherche du Temps Perdu, vol. XI (London, 1969)Google Scholar; Albertine Disparue p. 17 (tr. Scott-Moncrieff).Google Scholar

¹² However Nagel once told me of a man who could do just this.

¹³ Fingarette, in his book on the subject (Self-Deception (New York, 1969))Google Scholar goes so far as to conjecture that this phenomenon could be attributed to the fact of brain duplication. But it seems to me that there is no evidence for this hypothesis.

¹⁴ See Prince, M., The Dissociation of a Personality (London, 1905).Google Scholar

¹⁵ This case was, I believe, first discussed by Levy, J., Information Processing and Higher Psychological Functions in the Disconnected Hemispheres of Human Commissurotomy Patients (unpublished doctoral thesis, California Institute of Technology, 1969)Google Scholar. It is discussed by Nagel, op. cit.

¹⁶ There is no marked cerebral dominance in non-human animals, and so no reason for monkeys systematically to prefer one hemisphere/hand pair to the other.

¹⁷ See Trevarthen, C., ‘Functional Interactions Between the Cerebral Hemispheres of the Split-brain Monkey’, in Ettlinger, G. (ed.), Functions of the Corpus Callosum (London, 1964).Google Scholar

¹⁸ I know of only two cases: a man whose left hand seemed hostile to his wife, and who occasionally pulled his trousers up with one hand while pushing them down with the other—see Sperry, R., ‘Brain Bisections and Mechanisms of Conflict’ in Eccles, J. (ed.), Brain and Conscious Experience (Berlin, 1966), 298–313Google Scholar; and a man who pushed a plate away from him with one hand, towards him with the other—this case is cited by Puccetti, op. cit.

¹⁹ Sperry, , op. cit.Google Scholar

²⁰ As Sperry does himself; he writes: ‘We find the interpretation of this… to be less misleading if we do not try to think of the behaviour of the commis-surotomy patient as that of a single individual, but try to think instead of the performance capacities of the left and right hemispheres separately’. ‘Hemisphere Deconnection and Unity in Conscious Awareness’, American Psychologist XXIII (1968), 723–733.Google Scholar

²¹ Parfit, D., ‘Personal Identity’, Philosophical Review LXXX (1971), 3–27.CrossRefGoogle Scholar

²² This surprising argument is to be found in Puccetti, 's article (op. cit.), pp. 352–353Google Scholar. He describes two hemispherectomy patients, E.G. and G.E., and writes: ‘It seems clear… that if one could combine G.E.'s verbal and graphic skills with E.G.'s Gestalt abilities, we would have a normal human being. We would also have, given their complementary motor controls, an organism showing no signs of bodily paralysis. In other words, someone like you or me. But G.E. and E.G. are not half persons. If not, it follows that they were dual persons before hemispherectomy. G.E. was wrong to say she does not have both halves of her brain. She has her brain all right; it is just that she is no longer being helped out… by another person now gone because of a tumour.’

²³ I am grateful to many who have read or heard earlier versions of this paper, but particularly to M. Inwood, M. Lockwood, T. Nagel and D. Wiggins.