Split Brains and the Compositional Metaphysics of Consciousness


Luke Roelofs- Postdoctoral Fellow in Philosophy at the Australian National University

The mam­mali­an brain has an odd sort of redund­ancy: it has two hemi­spheres, each cap­able of sup­port­ing more-or-less nor­mal human con­scious­ness without the oth­er. We know this because des­troy­ing, inca­pa­cit­at­ing, or remov­ing one hemi­sphere leaves a patient who, des­pite some dif­fi­culties with par­tic­u­lar activ­it­ies, is clearly lucid and con­scious. The puzz­ling implic­a­tions of this redund­ancy are best brought out by con­sid­er­ing the unusu­al phe­nomen­on called the ‘split-brain’.

The hemi­spheres are con­nec­ted by a bundle of nerve fibres called the cor­pus cal­losum, as well as both being linked to the non-hemispheric parts of the brain (the ‘brain­stem’). To con­trol the spread of epi­leptic seizures, some patients had their cor­pus cal­losum severed while leav­ing both hemi­spheres, and the brain­stem, intact (Gazzaniga et al. 1962, Sperry 1964). These patients appear nor­mal most of the time, with no abnor­mal­it­ies in thought or action, but when exper­i­menters man­age to present stim­uli to sens­ory chan­nels which will take them exclus­ively to one hemi­sphere or the oth­er, strange dis­so­ci­ations appear. For example, when we show the word ‘key’ to the right hemi­sphere (such as by flash­ing it in the left half of the patient’s visu­al field), it can­not be verbally repor­ted (because the left hemi­sphere con­trols lan­guage), but if we ask the patient to pick up the object they saw the word for, they will read­ily pick out a key — but only if they can use their left hand (con­trolled by the right hemi­sphere). Moreover, for example, if the patient is shown the word ‘keyring’, with ‘key’ going to the right hemi­sphere and ‘ring’ going to the left, they will pick out a key (with their left hand) and a ring (with their right hand), but not a keyring. They will even report hav­ing seen only the word ‘ring’, and deny hav­ing seen either ‘key’ or ‘keyring’.

Philosophical dis­cus­sion of the split-brain phe­nomen­on takes two forms: arguing in sup­port of a par­tic­u­lar account of what is going on (e.g. Marks 1980, Hurley 1998, Tye 2003, pp.111–129, Bayne & Chalmers 2003, pp.111–112, Bayne 2008, 2010, pp.197–220), or explor­ing how the case chal­lenges the very way that we frame such accounts. A sem­in­al example of the lat­ter form is Nagel (1971) which reviews sev­er­al ways to make sense of the split-brain patient — as one per­son, as two people, as one per­son who occa­sion­ally splits into two people, etc. — and rejects them all for dif­fer­ent reas­ons, con­clud­ing that we have found a case where our ordin­ary concept of ‘a per­son’ breaks down and can­not be coher­ently applied. My work devel­ops an idea in the vicin­ity of Nagel’s: that our ordin­ary concept of ‘a per­son’ can handle the split-brain phe­nomen­on if we trans­form it to allow for com­pos­ite sub­jectiv­ity — some­thing which we have inde­pend­ent argu­ments for.

Start with what Nagel says about one of the pro­posed inter­pret­a­tions of the split-brain patient: as two people inhab­it­ing one body. Pointing out that when not in exper­i­ment­al situ­ations, the patient shows fully integ­rated beha­viour, he asks wheth­er we can really refuse to ascribe all their beha­viour to a single per­son, “just because of some pecu­li­ar­it­ies about how the integ­ra­tion is achieved”(Nagel 1971, p.406). Of course some­times two people do seem to work ‘as one’, as in “pairs of indi­vidu­als engaged in a per­form­ance requir­ing exact beha­vi­or­al coordin­a­tion, like using a two-handed saw, or play­ing a duet.” Perhaps the two hemi­spheres are like this? But Nagel wor­ries that this pos­i­tion is unstable:

“If we decided that they def­in­itely had two minds, then [why not] con­clude on ana­tom­ic­al grounds that every­one has two minds, but that we don’t notice it except in these odd cases because most pairs of minds in a single body run in per­fect par­al­lel?” (Nagel 1971, p.409)

Nagel’s worry here is cogent: if we accept that there can be two dis­tinct sub­jects des­pite it appear­ing for all the world as though there was only one, we seem to lose any basis for con­fid­ence that the same thing is not hap­pen­ing in oth­er cases. He con­tin­ues:

“In case any­one is inclined to embrace the con­clu­sion that we all have two minds, let me sug­gest that the trouble will not end there. For the men­tal oper­a­tions of a single hemi­sphere, such as vis­ion, hear­ing, speech, writ­ing, verbal com­pre­hen­sion, etc. can to a great extent be sep­ar­ated from one anoth­er by suit­able cor­tic­al decon­nec­tions; why then should we not regard each hemi­sphere as inhab­ited by sev­er­al cooper­at­ing minds with spe­cial­ized capa­cit­ies? Where is one to stop?” (Nagel 1971, Fn11)

Where indeed? If one appar­ently uni­fied mind could be really a col­lec­tion of inter­act­ing minds, why not think that all appar­ently uni­fied minds are really such col­lec­tions? What evid­ence could decide one way or the oth­er? Taking this line seems to leave us with empir­ic­ally unde­cid­able ques­tions about every mind we encounter.

What is strik­ing is that this way of think­ing isn’t prob­lem­at­ic for any­thing oth­er than minds — indeed it is plat­it­ud­in­ous. Most things can be equally well under­stood as one or as many, because we are happy to regard them sim­ul­tan­eously as a col­lec­tion of parts and as a single whole. What makes the split-brain phe­nomen­on so per­plex­ing is our dif­fi­culty in extend­ing this atti­tude to minds.

Consider, for instance, the phys­ic­al brain. Do we have one brain, or do we have sev­er­al bil­lion neur­ones, or even 8‑or-so lobes? The answer of course is ‘all of the above’: the brain is noth­ing sep­ar­ate from the bil­lions of neur­ones, in the right rela­tion­ships, and neither are the 8 lobes any­thing sep­ar­ate from the brain (which they com­pose) or the neur­ones (which com­pose them). And as a res­ult of the ease with which we shift between one-whole and many-parts modes of descrip­tion, we can be san­guine about the ques­tion ‘how many brains does the split-brain patient have?’ There is some basis for say­ing ‘one’, and some basis for say­ing ‘two’, but it’s fine if we can’t settle on a single answer, because the ques­tion is ulti­mately a verbal one. There are all the nor­mal parts of a brain, stand­ing in some but not all of their nor­mal rela­tions, and so not fit­ting the cri­ter­ia for being ‘a brain’ as well as they nor­mally would. And there are two over­lap­ping sub­sys­tems with­in the one whole, which indi­vidu­ally fit the cri­ter­ia for being ‘a brain’ mod­er­ately well. But there is no fur­ther fact about which form of descrip­tion — call­ing the whole a brain or call­ing the two sub­sys­tems each a brain — is ulti­mately cor­rect.

The chal­lenge is to take the same relaxed atti­tude to the ques­tion ‘how many people?’ Here is what I would like to say: the two hemi­spheres are con­scious, and the one brain that they com­pose is con­scious in vir­tue of their con­scious­ness and the rela­tions between them. Under nor­mal cir­cum­stances their inter­ac­tions ensure that the com­pos­ite con­scious­ness of the whole brain is well-unified: in the split-brain exper­i­ments, their inter­ac­tions are dif­fer­ent and estab­lish a less­er degree of unity. And each hemi­sphere is itself a com­pos­ite of smal­ler con­scious parts. This amounts to embra­cing what Nagel views as a reduc­tio.

There is some­thing very dif­fi­cult to think through about the com­pos­ite con­scious­ness view. It seems as though if each hemi­sphere is someone, that’s one thing, and if the whole brain is someone, that’s anoth­er: they can­not be just two equi­val­ent ways of describ­ing the same state of affairs. And this intu­it­ive res­ist­ance to see­ing con­scious minds as com­posed of oth­ers (call it the ‘Anti-Combination intu­ition’) goes well bey­ond the split-brain phe­nomen­on. It has a long his­tory in the form of the ‘sim­pli­city argu­ment’, which anti-materialist philo­soph­ers from Plotinus (1956, pp.255-­258, 342-­356) to Descartes (1985, Volume 2, p.59) to Brentano (1987, pp. 290­-301) have used to show the imma­ter­i­al­ity of the soul. In a nut­shell, this argu­ment says that since minds can­not be thought of as com­pos­ite, they must be indi­vis­ible, and since all mater­i­al things are divis­ible, the mind can­not be mater­i­al (for fur­ther ana­lys­is see Mijuskovic 1984, Schachter 2002, Lennon & Stainton 2008). Nor is the sig­ni­fic­ance of this dif­fi­culty just his­tor­ic­al: many recent mater­i­al­ist the­or­ies either stip­u­late that no con­scious being can be part of anoth­er (Putnam 1965, pp.163, Tononi 2012, pp.59­-68), or else advance argu­ments based on the intu­it­ive absurdity of con­scious­ness in a being com­posed of oth­er con­scious beings (Block 1978, cf. Barnett 2008, Schwitzgebel 2015).

All of the just-cited authors take the Anti-Combination intu­ition as a datum, and draw con­clu­sions from it about the nature of con­scious­ness — con­clu­sions up to and includ­ing sub­stance dual­ism. I prefer the oppos­ite approach: to see the Anti-Combination intu­ition as a fact about humans which impedes our under­stand­ing of how con­scious­ness fits into the nat­ur­al world, and thus as some­thing which philo­soph­ers should seek to ana­lyse, under­stand, and ulti­mately move bey­ond. As it hap­pens, there is a group of con­tem­por­ary philo­soph­ers engaged in just this task: con­stitutive pan­psych­ists. Panpsychists think that the best explan­a­tion for human con­scious­ness is that con­scious­ness is a gen­er­al fea­ture of mat­ter, and con­stitutive pan­psych­ists see human con­scious­ness as con­sti­tuted out of sim­pler con­scious­nesses just as the human brain is con­sti­tuted out of sim­pler phys­ic­al struc­tures. The most press­ing objec­tion to this view, which has received extens­ive recent dis­cus­sion, is the ‘com­bin­a­tion prob­lem’: can mul­tiple simple con­scious­ness really com­pose a single com­plex con­scious­ness (Seager 1995, p.280, Goff 2009, Coleman 2013, Mørch 2014, Roelofs 2014, Forthcoming‑a, Forthcoming‑b, Chalmers Forthcoming)? And this is at bot­tom the same issue as we have been grap­pling with con­cern­ing the split-brain phe­nomen­on. In my research, I try to explore the Anti-Combination intu­ition, its basis, and how to move past it, with an eye both to the gen­er­al meta­phys­ic­al ques­tions raised by con­stitutive pan­psych­ism, and to par­tic­u­lar neur­os­cientif­ic phe­nom­ena like the split-brain.



Barnett, David. 2008. ‘The Simplicity Intuition and Its Hidden Influence on Philosophy of Mind.’ Nous 42(2): 308­-335

Bayne, Timothy. 2008. ‘The Unity of Consciousness and the Split-Brain Syndrome.’ The Journal of Philosophy 105(6): 277–300.

Bayne, Timothy. 2010. The Unity of Consciousness. Oxford: Oxford University Press

Bayne, Timothy, & Chalmers, David. 2003. ‘What is the Unity of Consciousness?’ In Cleeremans, A. (ed.), The Unity of Consciousness: Binding, Integration, Dissociation, Oxford: Oxford University Press: 23–58

Block, Ned. 1978. ‘Troubles with Functionalism.’ In Savage, C. W. (ed.), Perception and Cognition: Issues in the Foundations of Psychology¸ University of Minneapolis Press: 261–325

Brentano, Franz. 1987. The Existence of God: Lectures giv­en at the Universities of Worzburg and Vienna, 1868-­1891. Ed. and trans. Krantz, S., Nijhoff International Philosophy Series

Chalmers, David. Forthcoming­. ‘The Combination Problem for Panpsychism.’ In Bruntrup, G. and Jaskolla, L. (eds.), Panpsychism, Oxford: Oxford University Press

Coleman, Sam. 2014. ‘The Real Combination Problem: Panpsychism, Micro-­Subjects, and Emergence.’ Erkenntnis 79:19–44

Descartes, René. 1985. ‘Meditations on First Philosophy.’ Originally pub­lished 1641. In Cottingham, John, Stoothoff, Robert, and Murdoch, Dugald, (trans and eds.) The Philosophical Writings of Descartes, 2 vols., Cambridge: Cambridge University Press

Gazzaniga, Michael, Bogen, Joseph, and Sperry, Roger. 1962. ‘Some Functional Effects of Sectioning the Cerebral Commissures in Man.’ Proceedings of the National Academy of Sciences 48(2): 17–65

Goff, Philip. 2009. ‘Why Panpsychism doesn’t Help us Explain Consciousness.’ Dialectica 63(3):289-­311

Hurley, Katherine. 1998. Consciousness in Action. Harvard University Press.

Lennon, Thomas, and Stainton, Robert. (eds.) 2008. The Achilles of Rationalist Psychology. Studies In The History Of Philosophy Of Mind, V7, Springer

Marks, Charles. 1980. Commissurotomy, Consciousness, and Unity of Mind. MIT Press

Mijuskovic, Benjamin. 1984. The Achilles of Rationalist Arguments: The Simplicity, Unity, and Identity of Thought and Soul From the Cambridge Platonists to Kant: A Study in the History of an Argument. Martinus Nijhoff.

Mørch, Hedda Hassel. 2014. Panpsychism and Causation: A New Argument and a Solution to the Combination Problem. Doctoral Dissertation, University of Oslo

Nagel, Thomas. 1971. ‘Brain Bisection and the Unity of Consciousness.’ Synthese 22:396–413.

Plotinus. 1956. Enneads. Trans. and eds. Mackenna, Stephen, and Page, B. S. London: Faber and Faber Ltd.

Putnam, Hilary. 1965. ‘Psychological pre­dic­ates’. In Capitan, William, and Merrill, Daniel. (eds.), Art, mind, and reli­gion. Liverpool: University of Pittsburgh Press

Roelofs, Luke. 2014. ‘Phenomenal Blending and the Palette Problem.’ Thought 3:59–70.

Roelofs, Luke. Forthcoming‑a. ‘The Unity of Consciousness, Within and Between Subjects.’ Philosophical Studies.

Roelofs, Luke. Forthcoming‑b. ‘Can We Sum sub­jects? Evaluating Panpsychism’s Hard Problem.’ In Seager, William (ed.), The Routledge Handbook of Panpsychism, Routledge.

Schachter, Jean-Pierre. 2002. ‘Pierre Bayle, Matter, and the Unity of Consciousness.’ Canadian Journal of Philosophy 32(2): 241­-265

Seager, William. 1995. ‘Consciousness, Information and Panpsychism.’ Journal of Consciousness Studies 2­3:272–2­88

Sperry, Roger. 1964. ‘Brain Bisection and Mechanisms of Consciousness.’ In Eccles, John (ed.), Brain and Conscious Experience. Springer-Verlag

Tye, Michael. 2003. Consciousness and Persons: Unity and Identity. MIT Press

Tononi, Giulio. 2012. ‘Integrated inform­a­tion the­ory of con­scious­ness: an updated account.’ Archives Italiennes de Biologie 150(2­3): 56­-90

2 thoughts on “Split Brains and the Compositional Metaphysics of Consciousness”

  1. I was won­der­ing if visu­al inform­a­tion was crosswired/contralateral like all the stuff con­nec­ted via the spin­al cord and dis­covered via web search it is cross­wired but by visu­al field as you cor­rectly put it not by say eye …

    I sort of thought the eyes might be dif­fer­ent from sens­ory input that goes through the spine because I have a com­prom­ised left inner ear but have more trouble bal­an­cing on my right leg sug­gest­ing that it is the left hemi­sphere get­ting the reduced inform­a­tion. My quick search sug­gests bal­ance is pro­cessed in the lower part of the brain and per­haps it is not cross wired like hear­ing (sort of is) and visu­al inputs are. Although no one says that.…

    I read Nagel recently and have heard about the strange phe­nom­ena of brain split­ting more inform­ally a lot. My biggest worry with all this is that philo­soph­ers may be doing a bit too much just respond­ing to each oth­er, some check­ing in (going back to the well) with the neuro-scientists and the exper­i­ence of split brain indi­vidu­als seems in order since dif­fer­ent research agen­das etc. can yield very dif­fer­ent per­spect­ives on the same phe­nomen­on. So some engage­ment with those more detailed and evolving per­spect­ives seems in order, oth­er­wise the whole thing seems in danger of becom­ing a pot­ted example that fails to show much.

    I would tend to say that this kind of thing is evid­ence against the pro­pos­i­tion that all men­tal life is some­how a irre­du­cibly unity, but I see lots of thing like that…

    1. Yes, the spe­cif­ics of sens­ory lat­er­al­isa­tion are rather fiddly. Nostrils to the ipsi­lat­er­al hemi­sphere, ears go to the con­tralat­er­al hemi­sphere, and the eyes are split by visu­al field (and even that’s a sim­pli­fic­a­tion). Even worse, touch for most of the body goes con­tralat­er­al, but touch for the head and neck seems to be handled by both hemi­spheres, and does­n’t show the same dis­so­ci­ations in the split-brain patient. I don’t know about bal­ance, though — I think there’s some evid­ence link­ing it to the cere­bel­lum, which is anoth­er whole kettle of fish.

      You’re right of course that philo­soph­ers should keep their eyes on the data and not simply on what oth­er philo­soph­ers say. If you think the split-brain phe­nomen­on is evid­ence against a spe­cially irre­du­cible unity in men­tal life, well then, we’re on the same side! But per­haps you don’t think that’s as import­ant a point to labour as I do?

Comments are closed.