Arguments from so-called “split-brain” phenomena are prevalent in the recent literature on the metaphysics of persons. Such phenomena are usually evoked, against the (Cartesian variety) dualist boogeyman, as proof of the neurological dependence of thought. A simplified, general, scheme can be given for such arguments:
1) If (Cartesian) dualism is true, then consciousness is brain-independent.
2) If consciousness is brain-independent, then the bifurcation of P’s brain should not result in the bifurcation of P’s consciousness.
3) But bifurcation of P’s brain (via commissurotomy) does result in the bifurcation of P’s consciousness
------------------------------------------------------
4) Therefore [from 2 & 3] consciousness is not brain-independent.
5) Therefore [from 1 & 4] (Cartesian) dualism is false
I tend to think that arguments in this general form are unconvincing. Particularly, I think, because premise (3) requires more argumentative support than is generally acknowledged. The trouble is that this premise appears to beg the question. In order to form the belief that the bifurcation of P’s brain results in the bifurcation of P’s consciousness, one must already believe that P’s consciousness is the kind of thing that can be bifurcated. On what grounds, other than a preexisting commitment to materialism about the mental, could one find support for this belief? Surely no one who holds an account on which P’s consciousness is taken to be a unified simple would be the least bit inclined to make such an assumption. But without this assumption, (3) cannot be true, and thus the argument from split brain phenomena fails.
I think many proponents of the argument from split brains recognize this. Accordingly, they hedge their bets somewhat and argue for the weaker claim that commissurotomy cases give us strong reason to believe in the neurological dependence of thought. I think even this much weaker claim is suspect. After all, this claim also begs the question against the dualist; insofar as it relies on the idea that something happens to P’s consciousness whenever P’s brain is split. Moreover, the argument for this weaker conclusion is inductive; at best it establishes a strong correlation between brain splittings and bizarrely disjunctive behavioral states. Torin Alter has recently argued, following Chalmers and Bayne, that as long as there is something it is like for the individual to be in the post-commissurotomy conscious state, then it makes sense to refer to them as being in a unified overall conscious state. This account may seem implausible, but surely it is neither any more implausible, nor any more open to empirical disconfirmation, than the thesis that a simple mental substance could itself be bifurcated.
4) Therefore [from 2 & 3] consciousness is not brain-independent.
5) Therefore [from 1 & 4] (Cartesian) dualism is false
I tend to think that arguments in this general form are unconvincing. Particularly, I think, because premise (3) requires more argumentative support than is generally acknowledged. The trouble is that this premise appears to beg the question. In order to form the belief that the bifurcation of P’s brain results in the bifurcation of P’s consciousness, one must already believe that P’s consciousness is the kind of thing that can be bifurcated. On what grounds, other than a preexisting commitment to materialism about the mental, could one find support for this belief? Surely no one who holds an account on which P’s consciousness is taken to be a unified simple would be the least bit inclined to make such an assumption. But without this assumption, (3) cannot be true, and thus the argument from split brain phenomena fails.
I think many proponents of the argument from split brains recognize this. Accordingly, they hedge their bets somewhat and argue for the weaker claim that commissurotomy cases give us strong reason to believe in the neurological dependence of thought. I think even this much weaker claim is suspect. After all, this claim also begs the question against the dualist; insofar as it relies on the idea that something happens to P’s consciousness whenever P’s brain is split. Moreover, the argument for this weaker conclusion is inductive; at best it establishes a strong correlation between brain splittings and bizarrely disjunctive behavioral states. Torin Alter has recently argued, following Chalmers and Bayne, that as long as there is something it is like for the individual to be in the post-commissurotomy conscious state, then it makes sense to refer to them as being in a unified overall conscious state. This account may seem implausible, but surely it is neither any more implausible, nor any more open to empirical disconfirmation, than the thesis that a simple mental substance could itself be bifurcated.
5 comments:
Have you read this paper already:
Neuropsychology and personalist dualism: a few remarks?
Titus Rivas
Titus,
thanks for the link (I'll check it out), but in the future please do not post comments on this blog unless it is to offer constructive philosophical engagement. Advertisements for other sites are not usually welcome. (This goes for all our readers.)
"as long as there is something it is like for the individual to be in the post-commissurotomy conscious state, then it makes sense to refer to them as being in a unified overall conscious state."
The descriptions of commissurotomies (whether through thought experiments or actual cases, in Sperry's work) also show that there is a unified conscious state:
"In working at this calculation I can see, from the movements of my left hand that I am also working at the other. But I am not aware of working at the other. So I might, in my right-handed stream, wonder how, in my left-handed stream, I am getting on." (Parfit talking about the possibility of splitting the brain to do calculations with different hands). Note that there is still a unified "I" switching between streams.
And even Roger Sperry who originally argued that consciousness was split admits that in actual neurological cases, they have to set up the studies in ways to prevent the hemispheres from sharing information. (Granted information-sharing <> consciousness)
Robert Sperry, “Consciousness, Personal Identity and the Divided Brain” Neuropsychologia, Vol. 22, No. 6, 669 (1984).
Also bodily consciousness (proprioception etc) remains intact and unified.
Just a few additional thoughts, even though I do think that thought is neurologically dependent and wouldn't go down the road of talking about mental substances a la Cartesian dualism.
I don't like premise 3 either, Adam. It's my sincere hope, though, that no published materialist philosopher would ever make such a claim, because what premise 3 says is either wrong or nonsense.
You seem to be portraying materialism as if it implied all statements like this: An X-ing of S's brain must be accompanied by an X-ing of S's consciousness. Nobody thinks this. That would commit us to such treasures as: Poking a hole in the left side of Adam's brain should cause a hole in the left side of Adam's consciousness. Or: painting my optic nerve blue will cause me to see blue.
Here's an exactly analogous example. Consider ten workers building a house. Perhaps when discussing what the ten are doing, we will want to introduce a thing called "the construction process" for ease of expression. Now suppose that Annie, while roofing, has a sudden heart attack and falls to the ground. Does it follow that some part of the construction process has a heart attack and falls to the ground? You'll agree, I'm sure, that the answer is no. Likewise, suppose a union roofer replaces Annie, so the work becomes more expensive. Does it follow that Annie becomes more expensive? Obviously, no. None of this dissuades us from asserting that the building process is identical with the workers and their actions. In general, when we abstract a process or function from its physical substratum, we don't expect sentences containing one noun to be truth-preserving under swappings with the other.
This is exactly what's at issue with brain-splitting arguments. Consciousness is the function or process; the brain is the physical substratum; and it is linguistically perverse to expect gross correspondences between alterations of the one and of the other. Frankly, I have no idea what a "bifurcation of consciousness" even is. (Well, what I think it is is a misapplication of terms.) Are we really going to base metaphysical arguments on whether you think someone else suffers from such a thing? How do you know your consciousness isn't bifurcated? When I'm whistling or doing a crossword puzzle, aren't I relying on instructions from somewhere in my awareness that I'm not, well, aware of? (The pronoun "I" here is very troublesome, and lets in all kinds of mistakes and sophisms.) In reality, I think, consciousness isn't unified, bifurcated, or n-furcated for any n, because it's not a thing at all. You can't cut it, it doesn't have folds or creases or gaps, except metaphorical ones. And I hope we aren't letting half-baked metaphors run our metaphysics.
Yeah, the second and third premises are a little simplified. William's second paragraph about sums up the problem--it isn't a matter of taking it as an axiom that, if X happens to a brain, then exactly the same thing will happen to the consciousness, as premise 2 suggests. Speaking more generally, I'd write it as, if consciousness is brain-independent, physical changes to P's brain should not influence P's consciousness.
Exactly what effects one would expect to see on P's consciousness depends on what area is affected. As for the corpus callosum in particular now.
The two hemispheres in the brain are usually connected by the corpus callosum. The left hemisphere usually controls language, for example, and the right, more of the basic emotional reactions. Each hemisphere also controls the opposite side of the body--i.e., the right hemisphere receives input from the left eye, and controls the movement of the left arm. Because the two hemispheres communicate with one another by the corpus callosum, the two are usually coordinated. So, with it removed, there are a number of strange results:
"Studies have repeatedly found that, if a patient with callosal disconnection is blindfolded and has an object put into their left hand, they will not be able to name or describe it (Heilman 2002, p. 128). The sensory information received by the right hemisphere cannot be transferred to the language systems of the left. However, since the right hemisphere controls movements of the left side of the body, including the left hand, the person will be able to use that hand to draw the object, or select it from among a group of similar objects, if asked to do so (Newberg and D'Aquili 2001, p. 23; Feinberg 2001, p. 92) - even while remaining unable to explain what they are doing or why."
"Kenneth Heilman offers another, more concrete example, writing about the research of Dr. Michael Gazzaniga and his colleagues. In one experiment, they showed sexually suggestive pictures to a woman with callosal disconnection, flashing them only on the left half of a screen so only her right hemisphere could perceive them. The woman giggled and blushed, but when asked why she was doing so, she replied that she was thinking of something embarrassing (Heilman 2002, p. 129)."
" "More than fifty years ago a middle-aged woman walked into the clinic of Kurt Goldstein, a world-renowned neurologist with keen diagnostic skills. The woman appeared normal and conversed fluently; indeed, nothing was obviously wrong with her. But she had one extraordinary complaint - every now and then her left hand would fly up to her throat and try to strangle her. She often had to use her right hand to wrestle the left hand under control.... She sometimes even had to sit on the murderous hand, so intent was it on trying to end her life." (Ramachandran 1998, p. 12)
"The obvious explanation was that she was mentally disturbed and doing this to herself, and indeed that was the diagnosis of several physicians who had previously examined her. But Dr. Goldstein found no signs of hysteria or other mental disorders - it genuinely seemed as if her left hand had a will of its own - and so he proposed a radically different explanation. He theorized that the woman's right hemisphere (which controls the left side of the body, including the left hand) had "latent suicidal tendencies" (ibid.) In a normal person, the more rational left hemisphere would inhibit these and prevent them from being translated into action; but if this woman had suffered damage to her corpus callosum, these inhibitory messages could no longer be transmitted to the other half of her brain, and the right hemisphere would attempt to act on its irrational self-destructive urges.
"Shortly after visiting Dr. Goldstein, the woman died (no, not from strangling herself). An autopsy confirmed the doctor's suspicions: she had suffered a stroke that had damaged her corpus callosum and severed the connection between the hemispheres, removing the brake her left hemisphere had put on the actions of her right." (source)
So, this is about what "bifurcation of P's consciousness" means--there's a variety of abnormalities that happen when the left and right hemispheres of the brain cannot communicate as usual.
That said, I disagree that the third premise begs the question. Instead, it describes the empirical results. You asked: "In order to form the belief that the bifurcation of P’s brain results in the bifurcation of P’s consciousness, one must already believe that P’s consciousness is the kind of thing that can be bifurcated. On what grounds, other than a preexisting commitment to materialism about the mental, could one find support for this belief?" We don't need to assume very much--it's just the results that we find when we look at people who have sustained localized brain damage.
I'd like to emphasize, that this basic argument can apply to many more areas of the brain than only the corpus callosum. There is, for example, the Phineas Gage case: in 1848 an accident damaged his ventromedial prefrontal cortex, an area important in social reasoning. Before this accident, he was considerate, and a hard worker. After, he became impulsive, profane, and lazy--a complete change in his personality. If premise 3 requires more support, there is an abundance of empirical evidence.
Post a Comment