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

Plantinga on Dualism


I’m in the process of writing the next podcast episode on Alvin Plantinga and his arguments around the idea of belief in God as a properly basic belief. In it, I’m clearly on Plantinga’s side, and I think his work in that area represents a crucial contribution to philosophy of religion (and to epistemology).

I thought, therefore, that I should balance that with the following rather less friendly assessment. I’ve mentioned before that I think that Plantinga’s defence of mind-body dualism is his major weakness (maybe his ontological argument for the existence of God is is second major weakness). Here he is briefly defending mind-body dualism in a talk with Robert Kuhn:

Argument: “It seems to me to be perfectly conceivable that I should exist when my body doesn’t.”

Now, it’s clear to me what Plantinga is getting at, and it’s equally clear that Kuhn has trouble understanding it. Most of Plantinga’s critics at Youtube who watched this clip have the same problem that Kuhn does. Kuhn is getting tangled up in the idea that if something is “possible” then everything in discussion is relegated to a possible world, so we don’t even know if the possibility exists in this world. One of the commentors at Youtube who – ironically – felt that he was competent to assess the philosophical quality of the argument rejected it, saying: “Worst goddamn philosopher ever. Because I can imagine it it’s? true…? .” This is (obviously) wrong, and Plantinga’s argument is a lot better than that. Plantinga’s argument is as follows:

  1. If I am identical with B (my body), then absolutely every true statement about me will be true of my body as well (since they are the same thing)
  2. “This thing can possibly exist without B” is a statement that’s true of me, but it’s clearly not true of B
  3. Therefore I am not identical with B (my body)

It’s not the worst argument in the world, granted, but an argument needs more than that going for it in order to be a good one.

The thought experiment derived from the story Metamorphosis has a couple of problems. For one, beetles don’t have eight legs, they have six. But that’s not the elementary error that matters here – it’s a different elementary problem. Is it really conceivable that I might exist when my body doesn’t? Plantinga’s language reveals a degree of question begging when he says that someone in the story woke up and found himself “in the body of a beetle.” In? It sounds like that description just supposed that we exist independent of a body and can be “in” any number of different bodies. If this is what he really means, then of course only a dualist could find this example even remotely plausible. A physicalist will just look at this thought experiment, if this is what Plantinga means, and say “no, this example is no good. That’s just impossible.”

Of course, the word “metamorphosis” does not at all suggest what Plantinga suggests here. Metamorphosis involves physical change, so what we have here is not Plantinga existing when his body does not. Instead, Plantinga’s form has changed from an upright two legged type form to a black, shiny six legged form. It’s deceptively easy to imagine this scenario just because it’s easy to imagine yourself looking like a beetle. This ease of thought distracts us from the rather significant metaphysical question that we are overlooking, namely that of whether it’s possible to exist apart from our body.

Perhaps an example from the classic horror movie The Wolf Man will suffice to make the point. In the movie, Larry Talbot is the wolf man. He has the curse of the werewolf, and in the full moon he transforms into the terrifying beast that feeds on human flesh. But nobody would take a transformation like this as evidence that Larry Talbot isn’t even a physical being! For Larry, B still exists. It just happens to be a lot hairier than usual.

For Plantinga’s thought experiment to work, therefore, it has to be construed as a case where 1) B is destroyed, 2) there is no causal relationship between B and the beetle body, and 3) There is no truthful sense in which B is the beetle body. But given these constraints, what will the physicalist make of Plantinga’s claim that this scenario is perfectly conceivable? The physicalist will be well with her rights reply, “no, it isn’t conceivable at all.” Verbalising a scenario is not at all the same of really conceiving of it in all of its details. All that Plantinga is really conceiving of, the physicalist will say, is waking up and looking like a beetle.


Nuts and Bolts 007: The King James Only Movement


Episode 036: Alvin Plantinga and Properly Basic Beliefs


  1. I can imagine Santa Claus. In fact, I’m going to have a beer with him right now.

  2. Tim

    This is curious; I think I’m going to agree with the parts of Plantinga with which you disagree, and vice versa! Ahh well …

    Regarding the modal argument, there is a pretty wide literature on the connection between conceivability and possibility. It’s been a while since I published on the subject, and I would have to get my head back into the details to get into a long discussion on it, which right now I don’t have time to do. But consider it from this point of view: do you really think that Berkeley’s metaphysics is logically impossible? I should say that, as modal intuitions go, the intuition that Berkeley’s metaphysics is at least logically possible is about as strong as any intuition about something of that sort can get. And that is all that Plantinga needs for that step in the argument.

  3. The atheist missionary – I don’t think that gives nearly enough credit to Plantinga’s argument. He does not say that if something can be imagined then it exists. That’s the same kind of error that some of the Youtube commentators were making.

    Tim, what I’ve said is that in stating that what he conceives is himself existing without B, Plantinga is mistaken. In other words, he’s not really conceiving of that at all. He only thinks that’s what it is that he’s thinking of. He’s naming the items in his thought experiment incorrectly.

    I’m not denying the link between conceivability and possibility.

  4. Tim


    Right, I do understand you to be stating this. And I understand that a physicalist, to remain a physicalist, may feel compelled to make the same denial. But I do not think that the analysis you gave really justifies this claim. Your response to Plantinga, if successful, would undermine his stated positive reason for his claim, pushing us back to the epistemic status quo ante. But that is not the same thing as showing that Plantinga’s modal claim is false.

    The Berkeley example would, I think, do just as well. Are you also inclined to bite the bullet there and say that Berkeley’s metaphysics is logically impossible?

  5. Well, I think that showing that Plantinga’s attempt [at showing that his modal claim is true] is unsuccessful is enough for my purposes here – and I feel pretty sure that I’ve at least done that. I have not offered an argument that shows the modal claim to be false. The physicalist just has no reason to buy the modal claim that Plantinga makes – not because of anything Plantinga has said anyway. If the physicalist thinks that he’s all-matter, why would he suppose that he could possibly exist when that matter doesn’t?

    However, I’d probably have to think a little longer and harder before talking about Berkeley, partly because I’m less familiar with the ins and outs of his thought than I’d like to be, and partly because I’m doing some thingking about the distinction between [the possibility of being non-physical if we’re actually physical] and [the possibility of having been created as non-physical when we could have been created as physical].

  6. Tim

    Fair enough!

  7. Kenny

    I think Plantinga was simplifying his presentation a little. But the interview was a bit painful to watch as it was, given the difficulty the interviewer was having with following the argument. So I think that a simplified presentation was probably the way to go.

    I was a bit surprised to hear him talk as if conceivability entails possibility. But I suspect he was employing an epistemically loaded notion of conceivability, one that involves the faculty whereby we are able to just “see” that certain modal claims are true.

    I don’t think that his using the metamorphosis story was the best way to go, for precisely the reasons you suggest, however. There isn’t a clear enough distinction there between B’s turning into a beetle body and B’s being replaced by one. I think Plantinga’s replacement thought experiment is much more powerful. It does seem (broadly logically) possible to me that I could be sleeping soundly, dreaming of my materialist days gone by, while all of the atoms in my body are rapidly replaced one by one, too quickly for them to be assimilated as parts of my body (this is a simplified version of Al’s thought experiment). I could sleep through this whole process without my dream being in the slightest interrupted. It seems clear to me that I would survive this process. But my body would have been destroyed if that we to occur.

  8. Oh, I don’t know Kenny. Assimilation is possibly pretty fast. 😉

  9. Kenny

    As long as it takes any time at all (and it does), Plantinga’s got it made.

  10. Jamale Nagi

    Plantinga’s argument is a good one– although perhaps dated. A clearer example would be Professor Xavier’s body being destroyed in X-Men 3–while his consciousness is transferred to other host (body). In this case one’s consciousness is A while the Body is B. In any possible world B cannot be B and not B at the same time. So to say the body exists when the body is destroyed is absurd. But its possible ala Xavier that the consciousness could exist while the body does not, in a possible world.

  11. Kenny


    The problem with that sort of argument is that it is not clear what a phrase like ‘my consciousness’ refers to. In fact, I’m inclined to doubt that phrase is genuinely referring at all. But, if it does refer, it seems to me that what it refers to are my psychological properties. And if that is what it refers to, my consciousness’s being transferred to another body while my body is destroyed is not sufficient for my surviving the process.

    Consider, for example, that a duplicate of me might be created (call him “Twin”), complete with my apparent memories and all the rest, *without* my body being destroyed (and without ceasing to have all the psychological properties that are currently associated with it continue to be associated with it). Pretty clearly, in that case, Twin wouldn’t be me. If I were annihilated a second later, I’d no longer exist, in spite of Twin still being around. But, then, how could it make a difference if Twin were created and my body were annihilated simultaneously? In that case, it also seems like I would no longer exist; I’d have merely been replaced by a duplicated.

    What is needed is a case that is (1) clearly (or at least clearly enough) broadly logically possible, (2) clearly such that my body is destroyed and (3) clearly such that I survive. In some of his other works, Plantinga has focused on cases that involve the rapid replacement of the parts of one’s body. I think that some of those cases do meet these criteria (at least well enough). I don’t think that they so clearly meet the criteria that a materialist can’t rationally resist the argument. But I think they meet them well enough to give an “all-else-being-equal” reason not to be a materialist.

  12. Jamale Nagi

    Thanks Kenny,
    I’ll chew on that a bit!

  13. “As long as it takes any time at all (and it does), Plantinga’s got it made.”

    How so? Does the replacement not take “any time at all”? If it does, then you’ve got to say either: That perhaps assimilation of parts as the process is happening can happen after all, since it takes the same or less time than the replacement of the whole body. OR: That replacement is more or less instant, and you just stipulate that there’s no assimilation going on. And of course, at this point the materialist can just wrinkle her nose and say that you’ve made it impossible for the person to survive after all. If there’s no assimilation, she may say, there may as well be a gap of one hour between the old parts disappearing and the replacement parts appearing.

  14. Kenny

    It just needs to happen more rapidly than the assimilation does.

    All that has to be the case is that it is broadly logically possible that the replacement time is shorter than the assimilation time. And surely it is. In the actual world, it takes time for new particles to become integrated with my metabolism in such a way that they become part of my body. Now just hold the time it takes fixed and imagine the replacement happening more rapidly than that.

    Suppose the replacement does occur faster than assimilation, but it occurs one atom at a time (very rapidly of course). Then the “replacement wave” could start at my left big toe, spiral around and work its way up while I’m dreaming happily. One could imagine that once the process got to my brain, it dances around whatever neurons and are firing at the moment, occurring only in dormant parts of my brain, while my neural activity shifts to different locations, making room for it, in such a way that there is no interruption in typical brain activity. By the time the whole thing finished, there’d be nothing left of my original body, but I’d have slept through the whole thing, dreaming happily without interruption.
    Now, I concede that the materialist can just dig in her heals here and say I wouldn’t survive. She could say that if that were to occur, my dream would abruptly come to a halt and that the person who awoke from the experience would be a mere physical and psychological duplicate of myself. And I concede that this may even be a rational thing for the materialist to say. I don’t claim for the argument that it is so compelling that a materialist must give in on pain of irrationality. I do claim that it gives an agnostic about the whole issue some reason to reject materialism. It does seem, absent materialist theoretical commitments, that I would survive.

  15. “It just needs to happen more rapidly than the assimilation does.”

    Yes, that’s basically a re-statement of my prior comment.

    And I see nothing here like “digging of heels” required. It’s not digging one’s heels in to observe that the dualist has stipulated that there’s no real continuity between one bady and the next, and therefor to conclude that it looks like a case of replacement. And if there’s no assimilation at all – which you are stipulating – then the old body may as well disappear for a few hours and then be replaced with a body made of chocolate.

  16. Kenny

    Glenn, there is (or at least can be) causal continuity (as the case is set up).

    We can imagine God intentionally swapping out the old atoms for new ones, for example. He’s putting the new atoms where he’s putting them because of where the old atoms were. What is the case is that the new atoms don’t have time to become *parts* of the old body, so that the composite object that is my body winds up getting destroyed. But, again, this happens one atom at a time, in such a way that none of my brain activity is disrupted (at least, in the same sort of way that the players in a basketball game could move to the side one by one and, one by one, be rapidly swapped out in such a way that the game play is never interrupted.

    It seems to me what would happen in that scenario is that I would experience a continuous stream of consciousness, unaware that anything unusual even happened. Intuitively at least, without letting theoretical prejudices get in the way, that’s what it seems would happen. Now, the materialist, letting theoretical prejudices intrude, can deny this. She can say that what would happen to me is that my consciousness would abruptly cease, and a duplicate would take my place. But what she is saying is a bit counterintuitive.

  17. Kenny, the only reason it “seems” to you that way is your prejudices, surely – just as for the materialist.

    I think your example trades on the lack of visual clarity about whether new parts are part of the old. The new body appears to most people to be made up from things that are part of the old. That’s the whole reason the replacement is not instant – so that it can look like the same body persisting. People are asked to imagine this in their minds, and what they see is a body losing partcles, which are replaced with others untilt he whole body has been replaced, and it looks like the kind of replacement of cells etc that happens right now, only faster. That is why they will agree “sure, this seems plausible.” That’s the origin of the intuition. But once you step in and say “but no – take note that this is not what is happening. The new cells are not part of the old body at all. It’s a total replacement, taking place in total before the new parts can be counted as part of the old body.” Once you’ve said that tot he observer, and they have thought about it and its implications, the intuition is seen to be grounded in a misleading appearance.

    Once you spell out to them that this appearance is misleading, the notion that you call counterintuitive just isn’t counterintuitive at all. In fact it seems natural to the materialist, just as it would be natural to deny that a person can survive being annihilated and replaced with something else that looks the same. I don’t see that it’s tenacious in any sense.

  18. Kenny

    Well Glenn, I think we may just have to agree to disagree on this one. Here’s someone arguing for your side:…/van-inwagen…/PlantRplArg4.doc

  19. Never mind ‘replacement times’ and God’s proposed intentionality. It’s very implausible indeed that all the molecules which compose my brain right now — I am seventy-eight* — are the same ones which went to make it up when I was eight.

    I’d go so far as to say that it’s highly unlikely that any of them are the same ones.

    Yet I am quite sure that I am the same person now that I was then.

    See? As so often, we don’t need appeals to abstruse or contorted philosophical thought-experiments. We need only look at the facts of the matter. They indicate — though they do not in themselves amount to a proof, I agree — that dualism represents the case and materialism is a load of old cobblers proposed by the philosophical equivalents of rather bright teenagers who want to look clever.


    Cheers and love,

    [* It happens, as a matter of marginal interest, that I was by around ten years Richard Dawkins’ senior at our mutual, excellent and quite unusually science-oriented UK public school, Oundle. It’s my hope that these ten years may persuade Dawkins, who is a first-class popular exponent of science and hence, I would judge, a pretty intelligent bloke, to look a little further into his current half-bright philsophical position and even — as Antony Flew has done — to repent of, and reverse, it. ]


  20. Martin, that your molecules are not the same ones that you had when you were eight surely tells us nothing in respect of dualism. That would suggest that dualism is true not just for humans, but for cats and dogs too.

  21. Why do you think that dualism is untrue for cats and dogs?
    Is it your belief that a cat is not (among other things) a non-material person? If so, why?

    I assure you my cat Josie is a person. Not a humanlike person, of course; a cat-person. (And no, I have no opinion upon the question of whether a mosquito is or is not a person; the answer in my view being that if God has provided a mosquito — or a bacterium, or a chair — with a personality then it is a person; if not, not.)

    [ Yes, I agree that my point that the molecules which compose my body now aren’t the same as those which composed it fifty years ago is not necessarily evidence for dualism.

    I didn’t intend it as such; merely as a counter to somewhat wild suggestions that “God might decide to change this or that physical aspect of our bodies and would that mean that we were no longer ourselves . . . ?” etc. The answer being: “No, it wouldn’t, because it happens anyway, and in the absence of any theoretical and supposed magical intervention by God.” ]



  22. What I mean is, your observation about your brain cells makes ALL living things dualistic. Cats and dogs yes, and also insects and plants too. After all, over time a plant could replace all its bits and pieces but still remain the same plant, but that doesn’t imply dualism.

    The intention of the “sudden repalcement” example is that it tricks people into thinking that there’s continuity of the person from one body to the next, when actually in the fine details, it turns out to be complete annihilation and replacement.

  23. You’r quite right. Physical replacement, whether sudden and magical or gradual/natural, tells us nothing whatever about dualism, which is the proposal that an entity — whether a chair, a cactus, a mosquito, a cat or a human being — has two actual (that is, not merely descriptive) components, one of which is physical (and is accordingly both subject to and completely described by physical laws) and the other immaterial, hence non-physical, hence not thus described and controlled.

    I am of course a dualist. But that doesn’t mean I consider a chair or a cactus to have two such actual components; whereas I am sure that every human being does so (and I presume, though with somewhat less certainty, that my cat does, too.)


  24. bedwetter777

    Horrible, horrible argument, Plantinga.

    3:50-in the video… But we don’t know if it is actually possible that he can exist when B doesn’t. So, we can’t say that, in actual reality, there is at least one thing true of him that is not true of B. It is possible his saying that he can possibly exist without B is merely an artifact of ignorance.

    Lois Lane can imagine Superman existing when Clark Kent doesn’t, but she can’t imagine Clark Kent existing when Clark Kent doesn’t. So, Superman and Clark are not the same person? Superman would disagree because he knows the true reality of the situation. Superman and Clark Kent are simply aliases of the person born Kal-El.

  25. Andrew


    I don’t quite see the problem in supposing that perhaps cats and dogs have souls as well. You throw this out as an obstacle to the dualist, yet it scarcely implies an incoherence to embrace the “reductio”.

    I do not see how it follows that Plantinga’s reasoning allows for the duality of plants. You’ll have to explain that one to me.

  26. Andrew, I don’t think I suggested that dualism for our pets makes dualism incoherent. I said it only because historically dualists have not believed in immaterial immortal souls for cows, dogs, ducks, fleas and dust mites (and Plantinga’s argument for dualism would apply to all of these). For any dualist of this sort, this speciic implication of Plantinga’s argument will give them pause.

    As for the issue of plants, Plantinga’s argument works for plants just as well as it works for humans. If Plantinga’s argument is not objectionable, then a cactus can have all its parts replaced very quickly and become an apple tree while remaining the same entity, which implies that the cactus is not identical with a physical thing – and this is what Plantinga used to show that we must be immaterial after all.

    So if this argument shows that you have an immaterial soul, it also shows that grass has an immaterial soul.

  27. Andrew

    I’m only concerned with what dualists have historically said insofar as they perhaps help me to develop a form of dualism that succeeds where theirs fell.

    I think that the distinction to be made between animals and plants is that, quite evidently, one is conscious and one is not. The dualist is not committed to saying that all living things are a duality, only that conscious entities are. The argument is that CONSCIOUSNESS is an immaterial property.

  28. Andrew

    (Note: I scarcely think that the fact that the modal argument might also work for animals is a problem for Plantinga. I happen to have an MP3 of a talk he gave a while ago wherein he “bit that bullet” and admitted that cats and dogs have souls)

  29. “I think that the distinction to be made between animals and plants is that, quite evidently, one is conscious and one is not.”

    Maybe, but the modal argument here doesn’t require consciousness, does it? Moreover, are dust mites and worms conscious?

    The point is just that when it comes to the beetle illustration Plantinga used, it works as well for the beetle as it does for him. If it shows that he is not essentially physical, then it shows that a beetle is not.

    Of course, it shows neither. All it shows is that we do not necessarily have to be in the shape that we are now.

  30. Andrew

    I should think that the modal argument does require consciousness since it tries to show that the property of consciousness is not identical with the brain.

    As for the question about dust mites and worms, I really don’t know. I’m the wrong person to ask. A scientist, of some description, would be better at answering that than me. But if it turns out that the scientist says “yes” the dust mite or the worm is conscious, then I just guess i’d have to “bite that bullet”.

    The last comment seems to assume a “supervenience” thesis that is, well, implausible to say the least.

  31. No, the modal argument is meant to show that I am not identical with my body.

    My previous comment had nothing to do with supervenience. Plantinga’s modal argument was intended to show that I am not identical with my body. But all it actually shows is that my body is not necessarily configured the way that it is now, which is fairly unimportant.

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