As I’m in the middle of a podcast series on the nature of the mind or soul and its relation to the body or brain (or both), my interests in general have been hovering around the issue, so here’s a blog post to add to the mix.
Alvin Plantinga is one of my favourite philosophers, but when he gets it wrong, he gets it surprisingly wrong. In general I think his work is the kind of thing that many aspiring Christian scholars (myself included) should aspire to produce. One particular skill that he has is to create helpful (and sometimes highly amusing) thought experiments to make the point. But every now and then I find myself thinking “wait, what?” I’ve concluded that like many great scholars, Plantinga is brilliant in general, but he has the odd soft spot in the head, noticeable by their contrast with the rest of his head. The ontological argument is one soft spot. Another is an argument that he uses for Cartesian dualism.
In his essay “materialism and Christian belief” in the volume Persons: Human and Divine, edited by Peter van Inwagen and Dean Zimmerman (Oxford: Oxford University Press, 2007), Plantinga presents two argument for substance dualism. The second of these arguments is the argument from thought, which is an argument from ignorance that I mention in episode 26 of the podcast (part one of the series In Search of the Soul). The first argument is the one that I’ll briefly comment on here. He calls it The Replacement Argument. Here’s how it goes (“B” here means “my body”):
It seems possible that I continue to exist when B, my body, does not. I therefore have the property possibly exists when B does not; B, however clearly lacks that property. By Leibniz’s Law, therefore (more specifically, the Diversity of Discernibles), I am not identical with my body.
If Plantinga had stopped there, he would have justly been accused of flagrantly begging the question. Generally speaking, a physicalist is very unlikely to believe that he can exist after his body no longer exists. The above claim then will simply be rejected as untrue and the physicalist will be unimpressed and unmoved. Fortunately, Plantinga says more. The replacement argument is really an argument for the above claim. He asks us to imagine medical science being advanced to such a level where medical professionals have the ability to replace just any body part that is maimed or rendered useless with a prosthetic part. Now imagine, Plantinga says, that he is sitting down reading a newspaper, and suddenly within the space of a microsecond, every one of his body parts is vaporised and replaced with – well, replacements. In a sitation like this, B will no longer exist, but Alvin Plantinga, he says, will still exist.
The rather visible flaw with this is twofold. In the first place, Plantinga is intentionally excluding the brain from replacement here. So it’s false that all of B has ceased to exist. Secondly, and this will not be the focus of what follows, a physicalist who thinks that a person is his body will yet again simply be unimpressed and unmoved by the rather unqualified claim that the person who undergoes a radical process like this still exists with no less of himself than before. Indeed, if a person is identical with his body, there just is less of him than before if his body is reduced to the size of a head. Secondly, a physicalist, unlike a dualist, is likely to regard the brain as the centre of consciousness, so of course if a fully functioning brain survives, the conscious person survives.
Realising this, Plantinga moves in for (what is apparently supposed to be) the kill.
But what about my brain, you ask – it is possible that my brain be replaced by another, the brain that I now have being destroyed, and I continue to exist? It certainly seems so. Think of it like this. It seems possible (in the broadly logical sense) that one hemisphere of my brain be dormant at any given time, the other hemisphere doing all that a brain ordinarily does. At midnight, we can suppose, all the relevant ‘data’ and ‘information’ is ‘transferred’ via the corpus collosum from one hemisphere – call it ‘H1’ – to the other hemisphere – H2 – whereupon H2 takes over operation of the body and H1 goes dormant. This seems possible; if it were actual, it would also be possible that the dormant half H2, be replaced by a different dormant half (in the same computational or functional state, if you like) just before that midnight transfer; then the transfer occurs, control switches to the new H2, and H1 goes dormant – at which time it is replaced by another hemisphere in the same computational or functional condition. In a period of time as brief as you like, therefore, both hemispheres will have been replaced by others, the original hemispheres and all of their parts annihilated by God. Throughout the process I serenely continue to read the comics.
This suffices, I think, to show that it’s possible that I exist when neither my body nor any part of it exists.
It’s mostly because I usually admire Plantinga’s work so much that I cringe at how terrible this is. Firstly, this looks to me like a confused notion of replacement. Yes OK, the original matter making up the first brain hemisphere no longer exists, but the same brain still does. The new matter does not come into being after all the old matter is gone. Observe: Let L = left and R = right. Let L1 = “the 1st version of the left hemisphere of my brain.” In Plantinga’s thought experiment, the following represents the compositional states of my brain over a short space of time. The underlined and bolded hemisphere is the controlling hemisphere.
L1 + R1
L1 + R2
L1 + R2
L2 + R2
L2 + R2
L2 + R3
L2 + R3
And so on. There is always a link between the brain at any given point in time and the version of the brain immediately prior. So at no point is the whole brain actually replaced. Here is what Plantinga needs to argue is possible:
L3 + R3
L4 + R4 or L4 + R4
He has not argued for this, and to suppose that it is possible is surely to beg the question. This is essentially to say that a person could be completely physically annihilated all at once and then fully replaced all at once, while continuing to exist. Why should a materialist grant this as a possibility? In any event, the article does not describe such complete annihilation and replacement. Even in the example he presents, therefore, a materialist has no reason to accept that Plantinga continues to exist while his brain no longer does. Sure, there are many previous versions of his brain that no longer exist, but his brain never goes from existing to not existing.
I say all this setting aside the monstrously false suggestion – from the point of view of neuroscience – that a left hemisphere could very quickly adopt all the duties of a right hemisphere while the person experiences “serenity,” but I realise that this would detract from the main point. I cannot account for why Plantinga finds his argument compelling (apart, perhaps, from the fact that it is his argument, a tendency I am probably equally vulnerable to). It is a chink in what is usually usually brilliant philosophical armour.
- Plantinga on Dualism
- Plantinga at the Sci Phi Show
- Episode 012: Plantinga and Presuppositional Apologetics
- Episode 032: In Search of the Soul, Part 4
- The rise of dualism?