The blog of Dr Glenn Andrew Peoples on Theology, Philosophy, and Social Issues

Dualism: Plantinga’s soft spot


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.

Glenn Peoples


No, I am not an inerrantist.


David Bain and the meaning of a "Not Guilty" verdict


  1. Kenny

    Well Glenn, I don’t think this is a terrible argument. I actually think it’s a pretty good (though not irresistible) argument.

    You are ignoring Plantinga’s points about replacement time. He does not deny that his brain can survive having all of its original matter swapped out. But it takes a finite amount of time for new matter to become part of one’s body. If the replacement happens rapidly enough, prior to the required integration time, then the original brain is destroyed before the new matter becomes part of it.

    I think the thing for the materialist to say here is that, in fact, Plantinga does not survive this procedure. The guy left, after the whole thing is over with, is a person who has just come into being but falsely believes that he is Alvin Plantinga. I think the materialist bites a bullet if she says this, but, imho, it’s not too big of a bullet for her to bite. Still, the argument is good insofar as it forces the materialist to bite a bullet.

    As an aside, you are also wrong to represent Plantinga’s argument from thought as an argument from ignorance. Plantinga goes into some detail explaining why it is not.

    Oh, and BTW, I also think that the way that Plantinga presents the OA is pretty good too! There are things that I disagree with Plantinga about (I’m a compatibilist, for example), but we disagree about where his soft spots are! 🙂

    I think we can both agree, though, that he is a great philosopher, regardless of whatever soft spots he may have. (And, in my own case, he’s helped me see, on more than one occasion, that the soft spots I thought he had were really due to my own).

  2. “If the replacement happens rapidly enough, prior to the required integration time, then the original brain is destroyed before the new matter becomes part of it.”

    I think the only plausible way to read the process that Plantinga describes is that the process takes place “In a period of time as brief as you like.” But it’s still a period of time, and far from ignoring Plantinga’s comments on timing as you suggest, I regard them as crucial. Plantinga is supposing that integration (i.e. the process of the new hemisphere being up and running with the new data) happens within the timeframe he is envisaging. Anything else would not allow survival, even of a new individual.

  3. Kenny

    ‘Integration time’ here (Plantinga uses the term ‘replacement time’ instead) does not refer to the amount of time it takes for the information to be transferred to the new hemisphere, but to the amount of time it takes for the new matter to become a part of one’s brain. Plantinga’s point is that if you “replace” the parts of a material organism (or part of one) too quickly, you destroy the material organism (or that part of it) rather than merely swap out its previous parts with new ones. So, if it is possible for me to survive a process whereby all the parts of my body/brain are replaced too quickly for my body/brain to survive, it is possible for me to survive the destruction of my body. But if it is possible for me to survive the destruction of my body, I am not identical to my body. (Check out Plantinga’s discussion of this on pp. 104-105 of the article that you cited.)

  4. But Kenny, the process of a new part becoming part of the brain by being added to the existing hemisphere happens within a space of time “as brief as you like.” There’s no way this is zero time, so there’s no way we can say that the old brain has been completely destroyed at any time. There is always a link between old and new, however briefly it might exist. If Plantinga intended us to think otherwise (that the brain really has ceased to exist and yet the new person is the same one) then he’s just begging the question by assuming that this thought experiment really is possible in the broadly logical sense. I say that if the brain really did completely cease to exist (whether for a small part of a second or for an hour or a year), then the individual in question would simply die. So either he’s obviously wrong about whether or not the brain has ceased to exist, or else he obviously begs the question. Neither option is pretty.

  5. Kenny

    What can happen in a time “as brief as you like” is that the whole replacement process that Plantinga describes occurs. And if it can happen in a time as brief as you like, it can happen more rapidly than the integration time. So it can happen fast enough for the person’s brain to be destroyed.

    I don’t think that Plantinga is begging the question by presenting the argument this way. It may be that a sufficiently staunch materialist will reject the claim that it is possible for Plantinga to survive this process, but an agnostic about the issue might not. She may have the undefeated intuition that it is possible for Plantinga to survive this scenario. And given that she has that intuition, the argument may well rationally convince her (independently of her already being convinced of its conclusion) that Plantinga is not identical to his body (and, therefore, by generalizing from the arbitrary case, that human beings are not identical to their bodies).

    Likewise, a less staunch materialist, upon reflecting on this scenario, might also find that her intuition that it is possible for Plantinga to survive this process is stronger than her conviction that materialism is true, and the argument may rationally convince her for that reason.

    But if the argument can rationally persuade certain individuals to accept its conclusion who did not previously accept it, it is not question-begging.

  6. What? “And if it can happen in a time as brief as you like, it can happen more rapidly than the integration time.” The replacement time IS the integration time. A part has not been replaced until there is a new part in its place. It’s plain nonsense to talk about a part of the brain being replaced before there’s a new part that has become part of the brain.

    I don’t care that some people might be persuaded to overlook this, it’s still begging the question, and any who are persuaded to overlook it have simply been tricked. In fact my high view of Plantinga prevents me from thinking that he meant what you’re proposing. There’s just no way he could have suggested that a part of the brain could have been replaced until there’s a new part of the brain. Although it’s possible in the broadly logical sense for him to have meant this, he would not have proposed something so obviously false.

  7. Kenny

    Yes, I am using ‘integration time’ to mean the same thing that Plantinga does by ‘replacement time’ (in part to avoid the very confusion you mention here). Plantinga does not mean by ‘replacement time’ the time it takes for the whole process that he describes in his scenario to occur, but the time it takes for new “parts” to become integrated into his body (i.e. the time it takes for them to actually become a part of him).

    I agree that the way that Plantinga uses the term ‘replace’ and ‘replacement time’ is confusing. But I am confident that I am accurately interpreting the argument. Take a look at pp. 104-105.

    The argument, in broad outlines, is that it is possible for me to survive having all of the parts of my body swapped out before the new “parts” have a chance to become part of my body. So it is possible for me to survive the destruction of my body. So I am not identical to my body.

  8. Kenny

    Oh wait, Glenn, my mistake. I double checked. You’re right. Plantinga is using ‘replacement time’ in the way you suggest. He calls what I am calling ‘integration time’ ‘assimilation time’. Sorry for the confusion on my end!

  9. Kenny

    Here’s what Plantinga takes the relevant premise to be:

    It is possible that: the cells in B are replaced by other cells and then instantly annihilated while I continue to exist; and the replacement time for O and those cells is shorter than the assimilation time. (p. 105).

    Note that replacement time and assimilation time are distinct. In spite of my terminological confusion, the argument is as I have representing it to be. The “replacement time” is the time it takes to swap out all the old parts. The “assimilation time” is the time it takes for the new parts to become assimilated by Plantinga’s brain. If the replacement time is shorter than the assimilation time, Plantinga’s brain is destroyed. So if Plantinga lives through this, he is not his brain.

  10. But Kenny, while one half of the brain is replaced and then annihilated, the other half is the one controlling the body. Then at midnight the data transfer takes place. By this stage assimilation must have happened or data could not be transferred. Then after this the (formerly controlling) hemisphere is annihilated and replaced, and then another data transfer occurs, and so forth. There is a very very short time between all these events, but this is the order in which they occur. The replacement must be complete before assimilation can happen, or there would be nothing to assimilate (how can you assimilate something that hasn’t even arrived yet?) The upshot of this is that there is no point when the brain goes from existence to nonexistence while Plantinga survives. It’s not just wrong, but in my view it’s just stark raving mad to think that a new brain hemisphere could be assimilated to the other half of the brain before the new hemisphere has even arrived. Replacement must be completed first.

  11. Kenny

    Glenn, I’m not sure I see you objection. The new hemisphere does have to be there for the information transfer to take place. But it doesn’t have to yet be part of Plantinga’s pre-existing brain for the transfer to take place –i.e. it doesn’t have to have been assimilated yet. The biological assimilation process can be slower than the replacement and transfer process. And as long as both hemispheres can be swapped out before there’s a chance for assimilation and Plantinga can survive, Plantinga can survive the destruction of his brain.

    Note ‘replacement’ in this context should not be read as expressing something that entails that the thing doing the replacement is a new part. A doctor might “replace” some of my skin during a surgery by giving me a skin graft. At the moment that he grafts on the new skin, he has “replaced” the old skin. But it may take some time after that before the new skin is actually part of my body (because it will take some time for my body to incorporate it as a part).

    Here’s some relevant quotes from Plantinga:

    But is it really true that this process of replacement would result in the destruction of B? … Well, speed kills. When a part (a cell, say) is removed from an organism and replaced by another cell, the new cell doesn’t become part of the organism instantaneously; it must be integrated into the organism and assimilated by it … Let’s use the phrase ‘assimilation time’ to denote the time required for the assimilation of the new part… There is also what we might call ‘the replacement time’: the period of time from beginning of the replacement of the first part by a new part to the end of the time of replacement of the last part (the last to be replaced) by a different part … What’s required by the Replacement argument (or at any rate what is sufficient for it) is

    Replacement It is possible that: the cells in B are replaced by other cells and then instantaneously annihilated while I continue to exist: and the replacement time for O and those cells is shorter than the assimilation time.

    pp. 104-105

  12. Kenny – you again?

    If assimilation is what you say it is, then the information transfer cannot occur until assimilation has taken place. This is because the new hemisphere needs to not just be present, but also connected up in all the right ways in order for the transfer to occur. If you’re going to say that assimilation is some thing over and above physical replacement and having all the right sort of connections, then it’s a fiction. If it’s not physical replacement and appropriate wiring etc so that it starts working, then it’s nothing at all. Physical replacement and correct wiring to make it the new hemisphere start working are necessary before the transfer can successfully occur. I think if Plantinga were to dig in his heels and just insist that we not agree with this, then he’s just asking people to accept things that aren’t so.

    I just don’t see how Plantinga doesn’t see this (or you, for that matter).

  13. Kenny

    Me Again! 🙂

    It is not at all obviously necessary for assimilation to have occurred in order for the information transfer to have taken place. It’s not clear, for example, that having the neural connections hooked up is sufficient for the two hemisphere’s to be biologically assimilated. Neural impulses could possibly be transferred over a spatial gap, so that all that is required for the necessary communication between hemispheres is that they be sufficiently close to each other. (Remember that all we need is broadly logical possibility here. The scenario doesn’t have to be biologically realistic for it to be broadly logically possible.)

    One could make this same point vivid in another way. It seems broadly logically possible, for example, that my brain could be in communication with another brain that is not part of my body by way of radio waves. And so it is possible for information to be transferred between brain hemispheres that are not part of the same brain. But if that is possible, then the replacement scenario that Plantinga’s argument requires is also possible.

    I agree that one of us is missing something. But I feel comforted by the fact that if it’s me, at least I’m missing the same thing that Plantinga is (and so I am in good company!).

  14. Kenny, I guess what matters then is just how “hooked up” the new hemisphere needs to be for it to be considered to have become part of the existing brain. I think that once the new hemisphere is sufficienty hooked up to function correctly and receive the new info, it has become part of the existing brain in every sense relevant here.

    I think I’m going to just have to add you to the list of people who believe wild and crazy things and leave it there. After all, you have time for the ontological argument!

  15. Kenny

    Well Glenn, if Plantinga is also on that list, then sign me up! I don’t think that anyone manages to study philosophy for very long without coming to believe at least a few crazy things!

    But I really don’t see the justification for your claim that if the information transfer can take place, the new hemisphere has become “become part of the existing brain in every sense relevant here.”

    First, I don’t understand what it would be for something to be a part of something else in one sense but not in another. Something either is a part of something or it isn’t. (It seems that if there were different senses in which something can be a part of something, there would be different senses in which things can exist as well. It seems that were that the case, for example, one could have a situation in which there are two objects that are, in one sense, part of some bigger object composed only of those two objects, but in another sense not part of any object. So there would be senses in which the bigger object exists and other senses in which it does not. But I find the claim that things can exist in some senses but fail to exist in others to be unintelligible).

    Second, if there aren’t different senses in which something can be a part of something else, then it seems possible that the sort of information transfer process that Plantinga’s argument requires could take place even if the new hemisphere isn’t yet part of the pre-existing brain. The communication between the hemispheres, as I pointed out, may well take place through a spatial gap. The new hemisphere needn’t have come into spatial contact with the preexisting hemisphere, it needn’t be in any way integrated into Plantinga’s metabolic processes, etc. But it seems that the sort of conditions that I have just mentioned (as opposed to merely being capable of sharing information) is what is required for a material object to be part of a biological organism (and this is reinforced by my point concerning the possibility of brains communicating with each other by means of radio waves).

    Given all of the above, it seems to me that if you deny it is possible that the hemispheres can communicate in the required way without being parts of a single brain, the burden of proof is on you.

    But, hey, it’s your blog and I’ve probably overstayed my welcome on this thread, so I will let the issue go here 🙂

  16. Plantinga’s example has plausibility because there’s no temporal gap and the info is transferred onto a part of the brain (a new part). Take that away and the appeal is gone. The only reason the example has appeal is because of precisely that impression: That of a brain that cycles through parts but continues to exist.

    A remote brain would not enjoy that type of plausibility precisely because everyone could see that it’s not part of the new brain.

  17. Platinga must have mentioned Pylyshyn in his argument. Surely.

    See Progressive replacement of the brain

  18. Dave

    OK, so let’s see if I have understod this:

    * Plantinga says that since we could keep existing even if our body doesn’t, then physicalism is false.
    * When challenged on the antecedent claim – that we could keep existing when our body doesn’t, he wheels out a body replacement example to defend that claim (He uses a bug, but you could use a werewolf transformtion, which is cooler).
    * But the body replacement example will be one of two things. EITHER a) It is total replacement without assimilation of the new body with the old, so that in fact the old one may as well be annihilated and the new one come into being, OR b) There will be assimilation, such that there is overlap, and the new body is really a transformed version of the old.
    * But if a) is what Plantinga has in mind, then Plantinga is begging the question, and if b) is what he has in mind, then that’s compatible with physicalism after all.

    If Plantinga were to realise this and jettison the example, then he’s simply left with the claim in dispute: That you can go on existing even without your body.

    If that’s what you mean, then thanks Glenn! It looks like a decent analysis to me, and I concur – this is a terrible terrible argument against physicalism.

  19. Dave, that’s it in a nutshell!

  20. Cody

    “The second of these arguments is the argument from thought, which is an argument from ignorance”

    I think that’s, at best, an uncharitable description of his argument. An argument from ignorance would be something like “X is true because I can’t see how it wouldn’t be…”. Or, more specifically, something like the evidential problem of evil where a noseeum inference is relied on. Plantinga, however, doesn’t even come close to doing this. He argues that it isn’t possible that material objects can think, and that we can plainly see this. So it would be like a person labeling an argument for how a circle cannot be a square as an argument from ignorance: it might be wrong (though, I don’t think it’s incorrect), but it certainly isn’t an argument from ignorance.

  21. Cody, I do agree that Plantinga does not describe his argument as an argument from ignorance. I did not mean to suggest that he views his own argument in that way. Instead, I was (very summarily, admittedly) noting that I think it boils down to an argument from ignorance. Plantinga says that it’s an argument from “intuitions” about what’s impossible. I know he says that he’s talking about properties, including the property of being able to think, such that it’s not possible for a material thing to have them. I get that.

    But what he really does in his chapter is to draw on the claim of Leibniz that perception is “inexplicable” as an effect of mechanical causes. As I read through his chapter, the standout points of his argument just consist of sceptical questions: How could it be that X could do Y? Now again, I know full well that he then states that he isn’t just being sceptical based on what we don’t know, and he claims that on reflection is just seems impossible, but that doesn’t strike me as much of an argument, and I really do think it’s fair of me to characterise what’s going on as an argument from ignorance.

    FWIW, I don’t think there’s much we can do about that ignorance. But of course, that argument wasn’t the focus of what I’m saying in this blog post. We may simply have to disagree about whether or not my characterisation of the argument from thought is right (although I think it’s right).

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