In a 2013 critical notice of Jeff McMahan's embodied mind theory of personal identity, Oritsegbubemi Anthony Oyowe argued that commissurotomy (split-brain) cases expose unresolved tensions in the theory and left open a cluster of questions: what principled criterion individuates minds at a time (synchronic individuation), how that criterion is grounded in the dependency between brain substrate, consciousness, and mind, how it composes with a diachronic criterion of personal identity, what measure and threshold of integration among mental contents separates one mind from two, and what a positive theory of one-person-two-minds cases implies for egoistic concern and responsibility. Later literature has produced substantial partial progress — most notably Elizabeth Schechter's theory that the split-brain subject is two conscious psychological beings that jointly constitute one person, a post-2017 empirical dispute over whether severing the corpus callosum divides consciousness, and formal integration measures such as the integrated information theory's Φ — but no accepted account answers the individuation question as a whole, and foundational disagreement persists as late as 2025. The problem remains open.

Background

In some severe cases of epilepsy, surgeons have cut the corpus callosum, the large bundle of nerve fibers connecting the two cerebral hemispheres; the resulting patients are called split-brain or commissurotomy subjects. In laboratory tasks, such a subject can identify a stimulus flashed in either visual half-field, but cannot compare stimuli presented one to each hemisphere ('cross-matching inability'), and each hand may act on information unavailable to the other hemisphere — behavior that suggested to early investigators such as Roger Sperry that each hemisphere sustains a separate stream of consciousness, a 'center of consciousness'. Thomas Nagel's classic 1971 paper 'Brain Bisection and the Unity of Consciousness' posed the resulting puzzle: after the operation there seem to be two centers of consciousness, yet before it there was one person, and the operation does not seem to create a second person, so either counting minds is not counting persons, or ordinary unities of consciousness are more fragile than we assume. A 'mind' here is the totality of a subject's conscious mental states and capacities at issue in personal-identity theory; 'synchronic' individuation concerns how many minds exist at a time, while 'diachronic' identity concerns what makes a person at one time identical with a person at another. Jeff McMahan's 'embodied mind theory' (The Ethics of Killing, 2002) holds that we are essentially embodied minds: the human organism insofar as it has the capacity for consciousness, roughly the functioning brain and the body parts causally necessary to sustain it. McMahan tied personal identity over time to psychological continuity realized in this physical substrate, and addressed split-brain cases by appealing to the integration of mental contents and to the (undivided) capacity for consciousness, at times wavering between the two grounds — the instability Oyowe's 2013 notice 'Courting the Enemy: McMahan on the Unity of Mind' set out to expose. Oyowe's critique leaves several issues unresolved: a systematic synchronic account of mind individuation reconciled with a diachronic identity criterion; the precise dependency between brain substrate, capacity for consciousness, actual conscious fields, and mind individuation; a threshold or measure of the 'degree of integration' among mental contents that fixes when two centers of consciousness count as one mind or two (and two minds as one person or two persons); a positive theory of 'two-mind persons' covering the allocation of egoistic (prudential) concern and responsibility; and the empirical questions — residual interhemispheric pathways, subcortical structures, and confabulation — that bear on which interpretation the data support. The modern debate has crystallized into rival models: the switch model of Tim Bayne, on which there is one conscious stream whose contents alternate between hemispheres in step with attention; the unified-agency model of Yaïr Pinto and colleagues, on which divided perception coexists with a single conscious agent; two-minds views in the tradition of Sperry, developed systematically by Elizabeth Schechter, on which the split-brain subject contains two conscious psychological beings; and the principle of phenomenal unity — that any two simultaneous experiences of one subject are experienced together — whose apparent violation in split-brain subjects is treated as a constraint on general theories of consciousness, including the integrated information theory (IIT), which measures a system's integration by the quantity Φ and predicts that consciousness divides when interhemispheric integration falls below within-hemisphere integration.

Problem Statement

The problem is to give a principled, empirically grounded account of what individuates minds, tested against the split-brain (commissurotomy) phenomenon, and to draw its consequences for persons. The account sought is the one that jointly satisfies conditions (1)-(4) below. The following cluster of questions remains open: (1) Synchronic criterion — what measure and threshold of integration among simultaneously existing conscious contents makes them contents of a single mind, on principled (not stipulative) grounds applying uniformly to ordinary subjects as well, and whether a split-brain subject accordingly has one mind or two; (2) Grounding — what metaphysical dependency relation links the brain substrate, the capacity for consciousness, actual conscious fields, and the individuation of minds, so that the synchronic criterion is explained rather than merely described; (3) Diachronic coherence — how the synchronic criterion composes with, or constrains, a diachronic criterion of personal identity over time (as in McMahan's embodied mind theory, on which a person is the organism insofar as it has the capacity for consciousness), so that the account does not deliver one verdict about how many minds are present at a time and an incompatible verdict about identity across time; (4) Normative consequences — what consequences follow for the allocation of egoistic (prudential) concern and moral responsibility if a person has two minds. The empirical record bears on which philosophical interpretation is supported: cross-matching inability together with the unified-agency findings of Pinto et al. 2017, cross-cueing (one hemisphere's overt responses supplying cues the other hemisphere can act on), subcortical and residual interhemispheric integration, and confabulation.

References

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