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Split Brains: No Headache for the Soul Theorist
DAVID B. HERSHENOV
Department of Philosophy, University at Buffalo, Buffalo NY, 14217 USA dh25@buffalo.edu ADAM P. TAYLOR Department of History, Philosophy and Religious Studies, North Dakota State University, Fargo, ND, 58108-6050, USA aptaylor@gmail.com Abstract Split brains that result in two simultaneous streams of consciousness cut off from each other are wrongly held to be grounds for doubting the existence of the divinely created soul. The mistake is based on two related errors: first, a failure to appreciate the soul’s dependence upon neurological functioning. Secondly, a fallacious belief that if the soul is simple, i.e., without parts, then there must be a unity to its thought, all of its thoughts potentially accessible to reflection or even unreflective causal interactions. But a soul theorist can allow neurological events to keep some conscious thoughts unavailable to others.
Introduction It is commonly thought that the phenomenon of the split brain delivers a decisive knockout blow to soul theories.^1 The phenomenon is well known to neuroscientists as well as philosophers of mind. Brain splitting involves a surgical procedure known as commissurotomy, which severs the neural fibers of the corpus callosum, resulting in either a partial or complete interruption of inter- hemispheric communication. As a consequence of the procedure, which was first developed as a means to treat epileptic seizures, the patients experience a bifurcation of consciousness into two apparent “streams-of-thought.” Materialist philosophers of mind have widely contended that these results contradict the supposed unity and simplicity of the soul. This will be unwelcome news to the many theists who believe their religious beliefs require they be immaterial or have an immaterial component.^2 But to continue with the boxing metaphor that began this paper, we don’t think split brains even help the materialist contenders win their bouts with dualists on points. The fact that the splitting of the brain results in two contemporaneous spheres of consciousness that are in some
ways inaccessible to the other is not grounds for denying that there is one soul involved, the same soul that was thinking the person’s thoughts before the brain splitting. The mistake is based on two related errors. First, such a position fails to appreciate the soul’s dependence upon neurological functioning. Secondly, such a mistake is grounded in a fallacious belief that if the soul is simple, i.e., without parts, then there must be a unity to its thought. Thus unity could consist in all of its thoughts potentially accessible to (self-conscious) reflection or, at least, unreflective (merely conscious) causal interactions. But a soul theorist can allow neurological events to keep some conscious thoughts unavailable to others. Our contention is that not only should theists realize that split brains aren’t a problem for their soul theories, but an extension of the phenomenon actually provides support for a divine creation account of the soul over the rival accounts of emergent dualists or Unger’s dispositional theory of the soul. As pointed out long ago by Parfit, if the two separated cerebral hemispheres can each give rise to conscious states then, barring technical problems, it should be possible to transplant each consciousness-supporting hemisphere into a different brain (Parfit (1984), 251-55). Assuming the two resulting thinking beings would be distinct agents and persons, then there will be a need for at least one new soul attached to one of the cerebral hemispheres. However, an account needs to be given why two more souls didn’t emerge or were disposed to interact with the separated cerebral hemispheres before the fission and transplant. The emergent dualist and the dispositional soul theorist need the hemispheres to somehow prevent the appearance of another soul prior to the fissioning and transplant without preventing the presence of the single soul correlated with the entire brain. The theist has the advantage of a less convoluted theory in which God bestows just one soul upon the typical person’s body and then bestows the souls needed in the fission and transplant scenario to ensure that there are two agents controlling their respective bodies. 3
Why maintain that each thought of one and the same soul should be (potentially) involved with every other thought of that soul? Well, it might be because the soul is not extended with parts that can be physically cut off from each other thus blocking communication. This guiding assumption is still spatial in that it treats the soul as if it is either point-like and all of its thoughts are in the same place, or the soul is an extended simple, spread out but without parts able to block access of one conscious state to another. Since they are all together at the same partless location, they must be involved with each other. Notice the spatial language of being co-extensive with a field in McMahan’s earlier quote. It seems that McMahan pictures the soul as being (at least somewhat) analogous to a spatial region. Such regions have clear boundaries, and for any such region, R 1 , all events occurring within R 1 must be co-located within R 1. Any events that are not co-located within R 1 , must be happening in another region, Rn. In a split brain scenario, certain streams of thought occur in ignorance of each other. A materialist conception of a thinking organism or brain with its spatial parts can easily make sense of thoughts cut off from each other. They are just realized in different parts of the brain, the connections between such segments severed. But a soul doesn’t have spatial divides, so all of its contents should be available to such an immaterial thinker. It would then seem that a split brain must involve a creation of a new soul or two. And since souls are simple, the original soul can’t split into two. So either the original soul goes out of existence or remains as one of the two resulting souls. But then when the corpus callosum is restored and unity regained, either we have a new soul which mysteriously has the contents of the predecessors or one of the two souls disappeared and its contents miraculously transferred to the other. Parfit sees the split brain phenomena as reasons to deny what he calls non-reductionist account of the person, the paradigm example being the Cartesian soul. The person is not something
over and above the brain, body and its thoughts. Parfit introduces a thought experiment involving two hemispheres that are each equally able to support the person’s full psychological profile, and a subject who has been equipped with a means of deliberately blocking the communication between hemispheres. When the subject is faced with a difficult physics problem, she decides to activate the device and pursue separate possible solutions to the problem with each hand, the right hand will work on one possible solution, the left on another. When the solutions have been reached, the hemispheres will be reunited and the subject will be able to recall both streams of consciousness. Parfit argues that it is mistaken to object that this picture ignores the necessity of the unity of consciousness. This is because Parfit denies any such necessity. He argues that consciousness is more like a river than a canal. It can divide and reunite as a river does while flowing around an obstacle. When the mind of the subject in the physics exam case divides, two separate consciousnesses are produced. Each consciousness is itself unified, and each is distinct from the other. And neither is the person. Parfit thinks for reasons such as these we do better to adopt a reductionist account of persons which redescribes facts about persons in which the world could be given an entirely impersonal description. He claims: Because we ascribe thoughts to thinkers, it is true that thinkers exist. But thinkers are not separately existing entities. The existence of a thinker just involves the existence of his brain and body, the doing of his deeds, and the occurrence of certain other physical and mental events. We could therefore describe any person’s life in impersonal terms. In explaining the unity of the life, we need not claim this is the life of a particular person … these claims are supported by the case where I divide my mind. It is not merely true here that the unity of different experiences does not need to be explained by ascribing all of these experiences to me. The unity of my experiences, in each stream, cannot be explained in this way. There only two
position. Magnets generate magnetic fields in virtue of the alignment of the micro-fields of their constituent iron molecules. But the magnet and the field it generates are not identical. This is shown by the fact that the magnetic field occupies a region much larger than the magnet does. Furthermore, once the field is generated it takes on sui generis causal powers, moving the magnet itself (Hasker (2002), 190). Similarly the brain produces a soul-field which gives rise to causal powers distinct from those of the brain (for instance allowing for libertarian freedom and the unifying of conscious states, both of which Hasker thinks raises difficulties for property dualism ). Zimmerman, also an emergentist, likewise maintains that the soul depends on the brain. He argues that once there is sufficient neural activity to give rise to consciousness, there will be a subject for that consciousness which is also generated (Zimmerman (2010), 146). Plantinga argues for a different account of the dependence of the soul on the brain. On his view, which presumes theism, souls are paired with brains by divine act. And while he admits that “appropriate brain activity is a necessary condition for mental activity” he resists the urge to identify mental activities with the brain activities they depend on (Plantinga (2007), 135). He points out that many activities (e.g. walking, mountain climbing, and digesting) depend on the proper function of the brain, but this alone does not make them activities of the brain and nothing else. Dependence is not identity. If it were, Plantinga argues, then, since brain activity depends on blood flow and the proper functioning of the lungs, we’d have reason to conclude that brain activities were really cardio- pulmonary activities. A third option for the dualist is Peter Unger’s dispositionally paired soul (Unger (2006), 151-
- Unger’s account of the soul’s relation to the person’s brain (or brains) begins, quite speculatively, in the “monistic plenum” that preceeded the primordial physical universe. He imagines a “Super Big Bang” which then splits reality into two realms. On the one hand there is the spatial realm which contains the physical universe, and on the other hand there is a non-spatial, but
spacelike, realm that contains immaterial simples.^5 The immaterial simples possess the requisite dispositions to pair with sufficiently complex “brainy” matter so as to manifest a singular consciousness. He imagines that these souls would have been waiting around for billions of years for their reciprocating material partners to take shape. Elsewhere Unger claims that all souls (not just human souls, but also animal souls) are equivalently rich in their dispositions to produce thought (in concert with the right matter). So how do we explain the differences in the quality of thoughts these equivalently rich souls in fact produce? He argues that difference between a cat and a human is that the human has a better brain to go along with his richly “propensitied” soul.^6 As he puts it the human has “got a grand piano” and the cat has “got just a darned kazoo.” This dependency upon our brains, also explains, Unger suggests, how damage to the brain effects thought. In order to exercise its rational powers to the fullest, the soul needs a well-functioning brain. As will be discussed in a moment, we believe that many critics of soul theories, and indeed many soul theorists, have failed to take seriously enough the proposition that thought depends upon the brain. Critics of soul theories, like McMahan and Parfit, fail to take seriously the notion that the thoughts of the soul are determined by the functioning of the brain. They seem to think of the soul as somewhat independent of the brain, rather like an omnipotent homunculus sitting at the controls of the brain and capable of controlling and correlating its states, regardless of what is going on within. And this mistake makes the soul theory seem weaker than it really is with respect to split brain phenomena. On our view, the brain makes a crucial contribution to the production of thought but it is not a thinker. A rough analogy for a materialist is that they believe the full materially person needs his eyes, but the eyes aren’t the perceiver. Just as we can’t see without our eyes, so is it that we can’t think without a brain (or a similarly cognitively functional substitute). As Aristotle and later Thomas observed, the eye is to sight as the body is to the soul. The soul is not empowered so as to actively shuffle about the contents provided by the brain without regard for how those contents are
Now onto our main defense which unifies the neurological dependence thesis with the inaccessibility to consciousness. The same soul can be thinking both streams of thought of the split brain patient. Parts, which the soul lacks,^7 are not needed to explain the division of thought, that is, a single subject with a divided mind. The dependency on the spatially dependent brain and its parts is enough. Souls are paired with brains. As a result they have access to thoughts subserved by that brain, and that access occurs in a manner unlike the manner in which they have access to another person’s thoughts. When the brain is split, the soul still thinks all the thoughts realized or subserved somehow by the brain, but those thoughts lack some of their standard interactions. This is a synchronic version of someone being unable to recall something for a period, yet perhaps later being able to. The soul’s dependence upon the brain explains why thoughts in the soul can be isolated from each other. The physical realizations in the brain are isolated from each other. The soul will not be entertaining thoughts independently of certain brain activities. One should avoid being misled by the soul’s simplicity to mean that all of its thoughts must be involved with each other. Perhaps, as we hinted earlier, such an error still involves something like spatializing the soul as a simple all in one place, perhaps the soul is being conceived as point-like or an extended simple without internal barriers. One then assumes that either the thoughts are “piled up” on each other or flow into each other. But that is a mistake; it is projecting the spatial relations of the brain onto the soul. The soul’s having no parts is no reason to claim that all of its thoughts are stored together in some sort of co- conscious causal interaction. The simplicity of the soul doesn’t mean that it can compare any thoughts it thinks. The soul’s comparing thoughts, like its thinking any thoughts, depends upon the brain. If the neurological structures are not properly related, neither will be the thoughts of the soul. The split brain prevents neurological interactions which prevents the soul not from thinking the
thoughts dependent upon different hemispheres, but from thinking those thoughts together in a way that they can interact and produce further thoughts that depend upon that mutual influence. The soul can only think certain thoughts that have a neurological basis. Destroy that basis and the soul cannot entertain thoughts. And here is the key notion of our thesis: connections between thoughts require connections between various parts of the brain. The mistake exemplified by McMahan is to think since the soul is simple, then it can reflect at any time on any of the contents in its “field of consciousness.” We contend that McMahan and others mistakenly picture the soul as having either a spatial, perhaps point-like location, or an extended simple without internal barriers. We call this the global workspace account of the soul.^8 According to this view, the soul acts as a sort of immaterial (though somehow spatial) blackboard receiving inputs and coordinating outputs. So long as all the inputs lead to the same location, (the global workspace to extend the metaphor), the soul should have access to all the mental contents realized by the brain at any time.
Fig. 1 Soul
A B
C
Figure 1 represents the global workspace view of the soul. Let A and B represent the physical realization in the brain upon which the mental states A* and B* depend, and let C represent the
Fig. 2 Soul
A B
C
Figure 2 shows the result of the brain being split in such a way that the cognitive contents of brain states A and B are no longer horizontally accessible to each other, but the same soul has access to both mental states A* and B. It then appears that there are two distinct and simultaneous conscious events occurring in the soul, an A event and B* event. It might seem that the soul should be able to generate C* on it’s own by filling in the relations between A and B and the missing C. But it cannot. This gives rise to the mistaken impression that the soul theorist will have to implausibly posit there are now two souls where before there was one. The soul is the subject of thought but it doesn’t by itself determine which thoughts influence other thoughts. It is not some immaterial thing which regardless of the state of the brain can freely choose to reflect upon and rearrange its contents, weigh them against each other, draw inferences etc., anything in the soul being able to causally influence any other thoughts or be thought. It isn’t that since thought A* is in the partless soul, and since thought B* is in the same partless soul, the immaterial thinker can reflect upon them both, compare them, adjust other thoughts in light of them etc. Rather the soul can think about A* and B, or think about C which takes into consideration A* and B*, only because there are physical connections A, B and C in the brain upon
which the soul depends.^9 On our account, the soul is more like an immaterial mirror than a global workspace.^10 What goes on in the brain, has mental correlates that are immaterially reflected in the soul. Readers might wonder how it could be the same soul if the soul is not individuated by the extent of the psychology accessible to it at any time? Well, one reason - but by no means the entire story of soul individuation - is that the soul is linked to that brain and will have thoughts subserved by that brain. When the brain’s pathways are restored, the soul thinking the thoughts dependent upon that brain will be able to think those thoughts in relations to each other. The thoughts will be known in that special first person way, whatever it amounts to, that distinguishes your thoughts from another person’s. When your brain is split there may be times that some of your thoughts have to infer some of your other thoughts in a manner somewhat akin to how you infer the thoughts of others. But the dependency upon the brain doesn’t mean the soul will always have to draw such inferences. Being connected to one particular brain means that its contents are tied to connections within that brain. When the brain functions as it should, the soul will have a greater interaction between its thoughts than when the brain is damaged. But there is just one soul, a soul dependent upon the brain, but nevertheless, the thinker of the thoughts. We don’t think readers should protest that we haven’t offered an account of how the soul and the body/brain are connected. The so-called “pairing problem” doesn’t trouble those working within a theistic framework. Materialists are wrongly trying to tie the hands of the theists, not letting them work within their preferred framework in which God creates the soul. There isn’t any pairing problem to explain if God has the creative power that he is standardly acknowledged to possess (Plantinga (2006), 132-134). Plantinga responds to Kim’s pairing problem challenge by claiming that the linking is accomplished in a manner akin to God just saying “Let there be light” and there is light or God thinking “Let Adam exist” and therefore Adam comes to exist. So there is no new problem
same. Unlike the case of the split brain in one body, the two disjoint bodies would seem to be distinct agents. Moor and Korsgaard note that the split brain in a single body rarely produces any frustration of agency that one would expect if there really were two persons connected to a single body. But there would clearly be two distinct agents after the cerebral hemispheres were fissioned and transplanted. Yet prior to the fissioning and transplant, the hemispheres can’t produce emergent souls or there will be two people where we want just one. So the presence of an adjacent cerebral hemisphere must somehow serve to prevent each hemisphere from giving rise to a soul. Yet this prevention must not prevent the soul from thinking the thoughts that are realized in the entire brain. This prevention puzzle of the emergent dualist provides some additional reason to think that God would instead provide the needed soul(s) for the transplanted hemispheres as well as the original soul of the earlier undivided brain. Secular philosophers with dualist sympathies might opt for Unger’s view of particular souls being disposed to be connected with bodies or brains. He thinks there is just one soul disposed to interact with certain arrangements of matter. But his account will suffer the same prevention problem that plagued Zimmerman’s emergent dualism. It seems that parts of your brain could support thought if they appeared in different skulls of different bodies after fission and transplantation, so there would have to be souls disposed to interact with them. But why prior to fission would matter beyond the boundaries of each cerebral hemisphere block or extinguish the souls disposed to interact with them post fission? Moreover, the Unger-inspired soul theorist, unlike the emergent dualist, has a time-indexed Ship of Theseus-like problem. Assume that all the matter that first composed your body gets gradually replaced and is later reassembled elsewhere. Supposedly there will be a soul disposed to the reassembled matter but now the propensities of the respective souls involve “memories” of sort, or rather are time-indexed in a way hard to fathom. The soul of the reassembled matter would
somehow have waited its turn, not becoming involved with the matter earlier. The soul of the reassembled matter isn’t your soul, which is still co-located with your replacement matter body. The soul theorist who accepts divine creation of souls will have a new soul created for the replacement body just as he posited in the split brain and transplant scenario. However, secular materialists might object that there is no need for either Unger or ourselves to posit a soul with or without a divine creator. First, the Ship of Theseus-like problem for persons will just have the reassembled person thinking when its brain comes into existence appropriately organized. The original person has survived as the replacement person since organisms can replace all of their matter if it is done so gradually. Secondly, the neurological dependency of thought shows that souls can’t think on their own so why posit them to think even dependent thoughts? Third, why can’t the unity of agency provide the secular materialist with a single fully material agent and subject of thought in the split brain case? If such unity is providing soul theorists like the authors of this article with a single agent in the split brain case, why can’t the materialist help himself to a simpler theory that eliminates the soul? Our response is that the single soul provides a single subject of thought whereas the materialist has no equivalent entity with which to individuate thinkers and agents. To see this we will turn to the reasoning that converted Unger from his materialism to dualism. And the same problem that inspired Unger’s dualism leads Zimmerman to defend a suspect overdetermination of a single soul, an overdetermination that doesn’t cohere well with the above prevention treatment required to deal with split brains. So our question has become why posit a soul that depends upon the brain rather than just have the brain or the animal with the brain as the thinker? There are many reasons that have been given to deny materialism. The reductionist’s seeming inability to explain experiential states in terms of the structural and functional explanatory accounts available to science is perhaps the most famous. But we are especially fond of one in particular that motivates Zimmerman (2010) and
countless equally good candidates to be the brain. Unaided materialism cannot offer us a palliative against the possibility of these additional candidate thinkers.^13 But the soul theory, especially as we have presented it, can do so. It tells us that brains alone are not sufficient for the production of thought. Only with the soul, which provides unity and subjectivity to its activities (or events) can the brain achieve thought. Unger’s solution to The Problem of the Thinking Many is to have a single soul disposed to pair up with any number of overlapping bodies. The emergentist solution is to have just one soul emerging from all the brains configured a certain way. The emergent dualism account is problematic because all the equally good overlapping physical candidates should be able to produce different souls. But Zimmerman argues that they overdetermine the same soul. This view strikes us as dubious. Moreover, materialists may think they can help themselves to Zimmerman-style overdetermination. If it works for soul, why couldn’t it work for a materialistic single candidate thinker overdetermined by many overlapping material objects? They might object that whatever relation is posited to connect the many bodies to one soul could just as well connect them to one (soulless) material thinker.^14 But we think it is ad hoc for either the soul theorist or the materialist to appeal to overdetermination to obtain a single thinker. Still, the situation is worse for the materialist. We don’t think the materialist can avoid too many thinkers by appealing to some relation between all the candidate brains and bodies and a single material thinker. For example, if all the material candidates overdetermine or constitute the same material person, why don’t all the overdetermining or constituting entities also have the capacity to think? There aren’t physically significant differences between them and the overdetermined or constituted thinker. Moreover, we can imagine many of the larger overlapping entities losing parts and becoming spatially coincident with the other entities that overdetermine or constitute a thinker consisting of the very same physical parts. But why can’t
the former think? This is the standard grounding objection to spatial coincidence, constitution and extensive overlap. Since materialists maintain it is things composed of matter that think, then it is hard to see why comparably arranged composites of matter should not be able to think. It is easier to defend a position that denies any material beings can think than try to insist that only one of the many overlapping or spatially coincident material beings can think. So it isn’t as philosophically contentious to claim that many material things that overlap connect to an immaterial soul that thinks than it is to claim that all but one of these physically indistinguishable or virtually indistinguishable material things can’t think. The relationship between thinkers and non-thinkers is far more mysterious on materialist grounds than it is in the dualist framework. Linking all the overlapping bodies to one thinking soul is very different than linking them to one thinking material thing. A thinking soul is a very different kind of entity than a material thinker. Conclusion We have argued that the split-brain objection to soul theory, as espoused by thinkers like McMahon and Parfit (and many others), fails because it relies on a pair of mistakes: first, it fails to take seriously the dependence of the soul’s thought on its bodily organ of thought (the brain). And second, it treats the soul, on account of its simplicity, as a sort of global workspace in which all the thoughts of the soul are necessarily unified and accessible regardless of the state of the brain. We’ve shown that once these mistakes are identified and assiduously avoided, a soul-theory can be stated which makes sense of the apparently contradictory data of the split-brain phenomenon. After motivating the need for a soul due to the challenge of Unger’s Problem of the Thinking Many, we offered theistic story about the soul’s origins to handle an extension of the split brain phenomenon. We claimed the theistic account does a better job than its immaterial rivals in explaining why there is just one soul in the ordinary case and two when the cerebrum is fissioned and the cerebral hemispheres transplanted.^15
