The book argues for three duality claims: one concerning split-brain consciousness, one concerning split-brain intentional agency, and one concerning split-brain psychology generally.
Each of the duality claims amounts to a claim about personal identity. If there are two centers or streams of conscious there must be two subjects of conscious experience; two centers of agency means two agents; two minds means two thinking things, that is, two thinkers. Defending the duality claims thus requires two arguments. The first, substantially empirical argument, is that split-brain psychology is importantly divided along hemispheric lines, in the realms of consciousness, cognition, and the control of action. The second, substantially theoretical argument, is that such divisions in psychological activities and actions entail multiplicity of psychological beings.
Let’s start with the overarching, substantially theoretical argument (which owes a significant debt to Shoemaker’s 1984 materialist theory of personal identity). Let us use the term thinker broadly, now, to refer to the psychological subjects and agents of mental states and activities of various kinds, so that a thinker is not just a reasoner but also a decision-maker and subject of experience and so on. What is a thinker? One possibility is that a thinker is just an animal with various (psychological) capacities, but that the individuation conditions for animals are purely biological and not psychological. If that is right, then since (I assume) split-brain subjects are unitary qua animals, they are unitary qua thinkers as well. The personal identity debates about split-brain subjects have always been in-house debates between proponents of psychological views of personal identity. It’s beyond the scope of this post or even really the book to defend psychological accounts of personal identity against animalist accounts; I’ll just note that my own reading and observation has led me to conclude that it’s difficult to be a thorough-going animalist in practice.
On a psychological account of personal identity, thinkers will be (at least substantially) causally defined. That is, just as to be a thinker is to be a thinking system, and the individuation conditions for thinkers will be the individuation conditions for thinking systems. Well, what is a thinking system? It’s a system that thinks, where thinking itself is causally defined. That is, a thinking system is one that operates in a certain way, or one whose causal activities are of such-and-such types. Many or most of these activities (on some views, all of these activities) will be defined in terms of the internal operations of such systems: what its internal states are and the way these states interact with each other. So, thinking is a collection of kinds of causal activities, and to be a thinker is just to be a system that thinks.
Split-brain surgery changes the causal activities of the brain. The 2-thinkers claim amounts to the claim that, after split-brain surgery, the activities that constitute thinking operate within each hemisphere system—within R and L, as I call them—more nearly than they do within the split-brain subject as a whole, S.
I’ll pause to note that I don’t take R and L to be the RH and the LH, respectively. I rather take R to be the entire split-brain subject minus the LH and L to be the entire split-brain subject minus the RH. Technically, even this may be a simplification—I doubt their boundaries can be quite so neatly delineated in gross neuroanatomical terms—but qua candidate thinkers, R and L are an improvement over the RH and the LH. Among other things, in terms of their intrinsic structure and capacities, R and L are each equivalent to things that we know to be thinkers: human beings who have undergone (respectively) left hemispherectomy (removal of the left hemisphere) or right hemispherectomy (removal of the right hemisphere). One cannot say this about a mere hemisphere.
The 2-thinkers claim thus amounts to the claim that after split-brain surgery, the causal activities that constitute thinking operate within each hemisphere system (within R and within L), though naturally they will thereby also operate literally within the split-brain subject as a whole, S, since S is composed of R and L. R and L are thinkers more so than is S, however, because if you look at the way RH mental states interact with LH mental states, that interaction is (generally) not of the kinds that constitute thinking. Intrahemispheric interactions are of the causal kinds known as thinking.
The next step of the overarching argument, then, is to specify the kinds of causal activities that constitute thinking. The book does this only in general terms, by noting certain very basic and reliable generalizations about e.g. how percepts give rise to beliefs, how intentions are formed, and so on. These are generalizations about how mental states can interact with each other. Our very concepts of percepts, beliefs, intentions, and so on, are grounded in these kinds of generalizations. For instance, beliefs are caused by percepts, and not vice versa. Of course this is just a generalization; believing can cause perceiving, too. But we should distinguish two ways in which believing can cause perceiving.
First, believing can cause perceiving directly: I believe that there is someone following me, as a result of which I come to hear someone following me (though no one is). Assuming that this is possible (that it’s not just the fear evoked by my belief that causes the misperception), all I’ll say is that such cases must be to some degree deviant. To discover that beliefs can cause percepts just as much as percepts can cause beliefs would be to discover that, well, there are no percepts or beliefs.
Second, believing can cause perceiving indirectly: I believe that my hairbrush is under the bed, which leads me to look for it under the bed, as a result of which I come to see my hairbrush under the bed. This is a non-deviant way in which beliefs cause percepts; indeed, if you pause to think about it, it is obvious that I am constantly perceiving things because of some earlier contribution from my beliefs. Yet when I initially said that percepts cause beliefs, and not vice versa, this second class of “exception” to that general rule probably didn’t occur to you—though it is hardly exceptional! Why is that?
It is because, when we think about mental states of various kinds, we think of them using causal generalizations that concern direct mental state interaction. Direct mental state interaction is interaction between mental states that is not mediated by action and perception. In the hairbrush case, the interaction between belief and percept is so mediated, and is thus indirect. In the book, I suggest that indirect mental state interactions are too heterogeneous to ground generalizations about kinds of mental state. After all, indirectly, beliefs cause percepts just as much as percepts cause beliefs. When we assume that percepts cause beliefs, and not vice versa, what we really mean is that percepts directly cause beliefs. To the extent that the identity conditions of percepts and beliefs are grounded in causal generalizations about percepts and beliefs and how they interact, then, they’re grounded in a less heterogeneous set of causal generalizations: generalizations about how they directly interact.
At first pass, then, the empirical basis of the 2-thinkers claim is this: that after split-brain surgery, direct mental state interaction is substantially confined to intrahemispheric interaction, while interhemispheric mental state interaction is predominantly indirect. Recall the example, from yesterday, of how you could get the split-brain subject (or, L) to finally say the word “Key”: by letting him (or, L) see that he (or, R) had manually selected the image of the key. And again the theoretical basis of the 2-thinkers claim is to argue that thinking—perceiving, believing, decision-making, etc.—consists of causal activities whose identities depend upon their capacity to directly interact. Thus thinking is revealed to operate more nearly within each hemisphere system after split-brain surgery than across such systems.
Shoemaker, S. 1984. Personal identity: A materialist’s account. In S. Shoemaker and R. Swinburne (Eds), Personal identity. Blackwell Publishing.