When a human being's two cerebral hemispheres cooperate to support one mental life at the level required for personhood, yet each hemisphere could be disconnected from the other and then sustain its own divergent person-level mental life, how many persons are there? The standard method of counting says one; Bajakian (2011) argued this is inaccurate and defended the Multiple Person View, on which such a subject is two persons sharing one body while their hemispheres cooperate. His own notes defer two narrower matters: how the counting should go if people have temporal parts, so that the number of persons at a time might depend on what happens later, and what to believe in actual cases, given that the simplifying assumption of a cerebrum that can support a mental life after detachment from the brain stem is contrary to what scientific research suggests. Whether his counting principle is correct — whether the Multiple Person View, the standard Single Person View, animalism, or a hybrid account gives the right count, and whether his arguments survive the empirical facts about hemisphere capacities in ordinary human brains together with the metaphysical assumptions they rely on (person-counts independent of future contingent events; cooperating half-brains as atomic parts) — remains actively contested. The counting question and its presuppositions remain unresolved in the literature.

Background

In some severe epilepsy cases, surgeons cut the corpus callosum — the large bundle of nerve fibers that connects the brain's two cerebral hemispheres — an operation called commissurotomy or callosotomy. The resulting 'split-brain' subjects led twentieth-century researchers, notably Roger Sperry's group, to discover that each hemisphere can process information and guide action with striking independence: stimuli shown to only one visual half-field may be named by only one hemisphere and acted on by only the opposite hand. Sperry himself drew conclusions about consciousness and personal identity from this (Sperry 1984), and philosophers such as Roland Puccetti (1973) argued that each hemisphere of a split-brain patient sustains its own person, while Thomas Nagel (1971) used brain bisection to probe whether consciousness is necessarily unified.

This empirically grounded puzzle feeds a central debate in the metaphysics of personal identity. Psychological-continuity views (in the tradition of Locke and Parfit's Reasons and Persons, 1984) hold that a person persists in virtue of overlapping chains of psychological connections; animalism (Olson, Snowdon, Carter) holds that each of us is a human animal, and that psychological facts do not determine how many thinking beings occupy a body. Both traditions are stress-tested by division and fission cases — thought experiments in which one brain's hemispheres are separated or transplanted into two bodies. These cases generate the 'counting problem': in a body sitting in a chair and thinking, how many persons — or even how many thinkers — are there? Animalists press the worry that psychological views imply 'too many minds' (one for the person and one for the animal); psychological theorists reply that animalism mishandles cases where the mind and the animal come apart.

Mark Bajakian's paper 'How to count people' (2011) sharpened the question for the realistic case in which two hemispheres cooperate seamlessly (as in an ordinary intact brain) while each could, if disconnected, support its own person-level mental life. He argued that the standard method of counting — which yields exactly one person in your chair — is inaccurate in such cases, and defended the Multiple Person View (MPV): a subject of this kind is two persons whose mental lives happen to be coordinated through the intact forebrain commissures. His case included two named arguments against the Single Person View (SPV), the view that such a subject is one person, and against animalist treatments of the same cases. In the Contrast Case, two half-brains (in vats, or in otherwise paralyzed bodies) cooperate temporarily to support one mental life, and an explosion then destroys one half-brain and its body, leaving a sole survivor: the argument turns on what matters in survival, on the SPV's inability to defend its claim about that survivor, and on the arbitrariness of saying that one candidate rather than the other continues to exist, together with the challenge that if a divisible person can go from two sets of psychological capacities to one, an atomic person should be able to go from one set to two (as when a hemisphere put out of action by a stroke is restored to action). In Division, the cooperating half-brains of a single body are disconnected and each supports a different mental life; on the MPV the two people who inhabit the single body at the start of Division are the two half-brained people who exist and are persons at its end. The argument presupposes two contestable metaphysical assumptions: that how many persons there are at a time cannot legitimately depend on future contingent events (such as whether the hemispheres are ever actually disconnected), and that two cooperating half-brains can be treated as atomic parts rather than as proper parts of one thing. It also depends on empirical matters: whether the disconnected hemispheres of ordinary human brains can in fact each sustain person-level mental life (given hemispheric asymmetries and their dependence on the brain stem, which is never divided), which is a question about neuroscientific fact. The paper does not leave its border cases open: its Appendix argues that the cooperating halves of an ordinary cerebrum belong to one person only if they belong to one person after they are disconnected, and that even the cooperating halves of envatted cerebrums (a brain kept alive in a vat and fed artificial stimulation rather than bodily input) whose hemispheres control separate bodies by wireless signal belong to two people; one of its notes adds that lacking the technology for whole-brain transplantation or such wireless control does not prevent us from asking and answering how we should count people if we had it. What the notes do concede is narrower: that the position on which person-counts depend on what happens later 'ought to be addressed—but only after determining how we should count people assuming that they do not have temporal parts', and that the simplifying assumption of a cerebrum detachable from the brain stem 'is contrary to what scientific research suggests', so that only once this divide is understood can we better decide what to believe in actual cases (the notes record the dispute between Sperry (1984) and Eccles (1980) over whether a disconnected right hemisphere supports its own person-level mental life). Whether the MPV's own commitments hold, and whether the SPV or animalism can be defended without ad hoc moves, remain live questions in the subsequent literature.

Problem Statement

For a subject whose two cerebral hemispheres cooperate to support one mental life at the level required for personhood, while each hemisphere could be disconnected from the other and then sustain its own divergent mental life at that same level, is the standard single-person count accurate, or is the Multiple Person View correct in counting two persons?

The question is which count a principle for counting persons yields in such cases — one, two, a hybrid count (such as two experiential subjects within one person), or no determinate count — and whether that verdict can be sustained against the Single Person View, the Multiple Person View, animalism, hybrid positions, and no-fact-of-the-matter positions. The metaphysical constraints, empirical considerations, and border cases below bear on this question.

Adjudicating this question in a principled, non-ad-hoc way turns on a general principle for counting persons in such cases, and on how that principle fares against the rival positions in the field — the Multiple Person View, the standard Single Person View, animalism, hybrid positions (such as two experiential subjects within one person) and, in the later literature, positions denying that there is any determinate fact of the matter about split-brain mind and personhood (Lycan 2022) — as well as against Bajakian's Contrast and Division arguments and the subsequent split-brain literature. It turns on the metaphysical constraints any such principle relies on: whether the number of persons at a time may depend on future contingent events (such as whether the hemispheres are ever disconnected), whether two cooperating half-brains may be treated as atomic, and what assumptions about persistence, temporal parts (the doctrine that a persisting thing is composed of distinct stage-like parts at different times), and constitution the account requires; the source itself holds that the future-dependent position 'ought to be addressed—but only after determining how we should count people assuming that they do not have temporal parts'. And it turns on the empirical application to ordinary human brains: whether disconnected ordinary hemispheres actually sustain person-level mental life (the Sperry/Eccles dispute recorded in the source's notes), hemispheric asymmetries, and dependence on the brain stem, where the source concedes that the detachable-cerebrum simplification is contrary to what scientific research suggests, so that what to believe in actual cases remains to be decided. The border cases the source works through — hemispheric transplantation and fusion, envatted brains (a brain kept alive in a vat and fed artificial stimulation rather than bodily input), brain-stem dependence, and mixed biological/technological control — belong to that same terrain: the paper argues them to hedged conclusions rather than leaving them open, so those conclusions are part of what an adjudication of the counting question has to reckon with.

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