In the last post, I argued for property dualism. I argued that consciousness was not reducible to physical phenomena. So far, I have not addressed what kind of things we are. As I said in my first post, experiences are experienced by a subject, and learning the non-physical nature of our experiences does not immediately tell us the nature of the selves which have these experiences. It is compatible with what I have argued so far that we are material objects (our brains, say, or our bodies) which also happen to possess a special set of non-physical properties. As I mentioned before, this form of mere property dualism would be enough to get the argument from consciousness for theism off the ground (for reasons we’ll see later). However, I am going to argue in the next two posts that I am in fact a non-physical soul (or, if one prefers, am composed of a non-physical soul and a physical body–though it will become clear in the next post why I don’t like this formulation). So are you, and so are the other conscious animals.
A Quick Theological Point
Before beginning the argument, I will note a theological point. It’s often thought that Christians, as such, are somehow doctrinally committed to the claim that human beings possess immaterial souls. This doesn’t seem to me to be correct. Plenty of orthodox Christian philosophers reject substance dualism in favor of the view that we are material objects of some sort. (Most notable for me is Peter van Inwagen, a former professor of mine who, in addition to being one of the most prominent Christian philosophers alive, sponsored my confirmation into the Episcopal Church.) The biblical view of the afterlife is an embodied one in which we are resurrected, not one spent as immaterial spirits, and so seems to me to be basically neutral between substance physicalism and substance dualism. And the Bible more broadly does not seem to me to really stake out a position on the metaphysics of created persons: there are some suggestive remarks, but philosophy of mind is not really the point. Admittedly, there are some important philosophical objections about how God could ensure that the reconstituted person in the afterlife is really me, if I am a material object, but it isn’t obvious to me that these are decisive.
So: it seems to me that Christians, as Christians, can basically be open-minded about whether substance dualism is true. (Naturalists as naturalists, by contrast, cannot: substance dualism would falsify their naturalism, unless they are willing to count immaterial souls as “natural.”) However, there is then a question about where this open-mindedness should lead us. I think it should cause us to adopt substance dualism after all. In this post, I will discuss a number of modal arguments for substance dualism. (“Modality” has to do with what could or couldn’t happen–judgments of possibility, impossibility, and necessity.) I will first discuss a family of modal arguments which I think face a certain important objection, and will respond to that objection. However, I’ll then discuss an unusual type of modal argument from Richard Swinburne which I think manages to basically sidestep the objection. In the next post, I’ll discuss two very different arguments for substance dualism–ones having to do with the nature of composite objects.
In the previous post, I defended a number of arguments which relied on the claim that all the physical facts could be the same while the facts about consciousness were different; from this the arguments concluded that consciousness must be something over and above the physical, i.e., that property dualism was true. Modal arguments for substance dualism usually rely on the claim that I could exist when neither my body nor my brain (nor any other physical object which was a candidate to be me) did, so that I must be a non-physical soul distinct from any physical object.
The most straightforward modal argument (one famously employed by Descartes) involves the claim that it is possible for me to exist without any body at all. Recall, from the first post, Avicenna’s floating man, who has never had any bodily sensations and therefore doesn’t think he has a body. Is there something inconceivable or incoherent in his supposing that he exists as a self without a body? Could he, just by thinking about it, ultimately deduce that he must have a body after all, even without ever experiencing any bodily sensations? Surely not. But in that case, there at least doesn’t seem to be any conceptual connection between having a self and having a body. But if it’s conceivable that the floating man exist without a body, isn’t the same true for me? For instance, it seems conceivable that, as Descartes considered, I could in fact not have any body at all, with all my bodily sensations being supplied to me by an evil demon. Or it seems conceivable that, as many people have really believed, I can continue to exist after my bodily death, whether or not I actually do. (Of course, there is the by now familiar question about the connection between conceivability and possibility. Don’t worry, I’ll address it below.) If I really can exist without my body, or brain, or any other physical object, then it seems that I must not be my body, or my brain, or any physical object, since it obviously isn’t possible for something to exist when it doesn’t. And if I am not a material object, I must be an immaterial object, namely, a soul. 
If you’re not yet ready to sign onto the idea of disembodied existence, you might consider Alvin Plantinga’s “replacement argument.” Here it is. Suppose that I’m here reading the news. Bit by bit, little parts of me are somehow being replaced, perhaps through the use of a teleporter or something. Maybe, for instance, one cell at a time is being replaced with a new, identical cell, with the original cell being carried away and annihilated. Suppose this happens for every cell in my body: by the end of the process, I am in exactly the same physical state that I would have been in without the process, but all of the original matter is gone. Perhaps we think my body could survive this. After all, cells are destroyed and created, and matter is taken in and expelled, all the time. However, Plantinga suggests that when a cell is replaced in the way described in this argument, there is a brief window before the cell becomes part of my body because it takes a moment for the right causal relations to be instantiated:
When a cell 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. What does this assimilation consist in? A cell in a (properly functioning) body is involved in a network of causal relations; a neuron, for example, emits and responds to electrical signals. A cell receives nourishment from the blood, and cooperates with other cells in various causal activities… At the instant the new part is inserted into the organism, and until it has begun to play this causal role (both as cause and effect), the new part is not yet a part of the organism, but a foreign body occupying space within the spatial boundaries of the organism. (Clearly not everything, nor even everything organic, within the spatial boundaries of your body is part of your body: think of the goldfish you just swallowed, or a tapeworm.)
Now suppose the process of replacement happens really really fast–so fast that the first replacement part has not yet been causally integrated by the time the final cell is destroyed. Now my original body has no parts: it’s parts are all destroyed, and there hasn’t been time to assimilate the new ones. So my original body, presumably, has been destroyed; what exists at the end is a new, identical copy of my old body. But is it possible that I survive this process–perhaps that, throughout the whole thing, I continue sitting there happily reading the paper, having an uninterrupted series of conscious experiences? Note that the question is not whether I would in fact survive the process. Dualism is compatible with the claim that it kills me. The question is whether I could. If so, then it looks like I can survive the destruction of my body, brain, etc., and so am not identical to any of them.
A Different Modal Argument
Here’s a final, body-switching based modal argument, inspired by one from Richard Swinburne. It seems conceivable that I–understood as a subject of experience–could have, in essence, lived your life in your body, and you mine. (We often try to imagine situations like this. When I try to “put myself in your shoes,” what I am essentially trying to conceive of is my perspective being yours, with your body and mental states.) Swinburne suggests that “Just the same arrangement of matter and just the same laws could have given to me the body (and so the behaviour and mental life) which are now yours, and to you the body (and so the behaviour and mental life) which are now mine.” But if we are physical objects–even ones with irreducibly mental properties–it isn’t clear how this could be possible: “…if mere property dualism were true and so mental properties were properties of brains or bodies, and there were no souls, there would be no difference between [the actual situation and the one where we are switched], since the same mental lives would be associated with the same brains (and so bodies) in both situations.” On the other hand, if we are essentially immaterial souls, it seems clear how this scenario could be possible: our souls might have been causally related to different bodies.
What to make of these arguments? The crucial issue will again be whether the dualist can defend the move from the conceivability of the situations in question to their real metaphysical possibility. However, modal arguments for substance dualism face a special wrinkle not faced by those for property dualism. It will not be enough for these thought experiments to show that there is no essential connection between being a person and being material, so that some possible person is a soul. Learning that immaterial persons are possible is itself interesting, but what we wanted to know was whether you and I are souls, not just whether any possible person is a soul. In presenting the arguments for property dualism, we were concerned with the question of whether one type of property was reducible to another. All we needed for that was the claim that it was possible to have a situation where the one type of property was instantiated while the other wasn’t. Here, we are concerned with the question of whether a certain concrete substance (me, or maybe you) is identical to another certain concrete substance (a particular brain or body or whatever). The technical way to express this is that one involves a de dicto claim while the other involves a de re claim.
Some materialists have argued that this is a special problem for modal arguments of the sort mentioned. They agree that someone could be an immaterial soul, able to exist apart from their body or any other material object. So in that sense, they agree that the above scenarios are possible, as long as we strip out the personal pronouns. But they claim that it doesn’t follow that you and I are immaterial souls; we (they claim to know on empirical grounds) are essentially material. And they suggest that, as we consider scenarios like those above, there is not really any way to tell whether we are actually conceiving of things which are possible for ourselves, or merely things which are possible for someone or other. If there are essentially material and essentially immaterial possible people, how can I know that the scenarios in question are possible for me unless I already know which type I am–in which case the argument is pointless?
Here’s the thing. When I began writing this post, I thought that this objection worked against the arguments discussed above. I included the discussion of these modal arguments in order to set up a contrast with the transplant argument I discuss in the next section, to show why I thought it got around the objection when these others didn’t. In the process of actually writing this post, I became a little less sure. Many of the issues here are difficult, and, after all, it does clearly seem that sometimes we are able to make modal judgments about particular concrete substances, not just general types. So I then decided I would end this section with a moderate conclusion: perhaps, for those to whom it seems like the above scenarios are possible for them, this seeming may provide some justification for accepting substance dualism–but the justification is tempered by the worry the materialist raises.
However, around the same time, I read Richard Swinburne’s recent book Are We Bodies of Souls? The book contains a discussion of Swinburne’s account of “informative designators,” which are supposed to help us get around the objection. I knew about Swinburne’s account previously, but had never really been able to figure out how it was supposed to work. However, having read this book, I now think I more or less understand what Swinburne is getting at–and that he may be right.
Your best bet here is likely just to read Swinburne himself (chapter five is the relevant one). Very basically, his idea is something like this (I will just try to give a sense of it, not a full defense). I suggested in the last post that each of us is in a position to know the nature of our qualia in virtue of experiencing them, and therefore being directly acquainted with their natures. And I suggested that this put us in a position to infer possibility from conceivability with a degree of confidence we couldn’t otherwise have–that this put us in a position to rule out there being some hidden impossibility in the zombie scenario. Swinburne can be thought of as arguing that each of us is similarly in a position to know exactly what it is for a certain experience to be our own, rather than someone else’s. And he thinks that this puts us in a position to know that each of us really is imagining a scenario which is possible for ourselves, and not just for some otherwise similar person, when we consider scenarios like those above.
As I said, I’m now much more sympathetic to the thought that something like this might be right than I was when I began writing these posts. However, I will not spend more time explaining or defending this idea. This is partly because I think Swinburne provides a particular modal argument which can plausibly succeed even if his general defense of modal arguments against the objection here fails. I present my own take on that argument in the next section.
The Transplant Argument
This argument is somewhat different from the other modal arguments, but could still be classed as a modal argument insofar as it ultimately relies on judgments about what sorts of things could or couldn’t happen to me. Here it is. Suppose the villainous scientist who removed my brain and did all that stuff with the people with walkie-talkies at the end of the last post retrieves my brain from the trash, fortuitously undamaged, and puts it in a new body, which we’ll title B. It seems fairly natural here to think that I–i.e., what I previously called my “self,” understood as the subject of experience–go along with my brain. My point of view is now associated with B. Well and good.
Now consider: we know from neuroscience that there’s a lot of duplication and redundancy between the two hemispheres of our brains: they can perform many of the same tasks, store some of the same memories, etc. Furthermore, thanks to “neuroplasticity,” one hemisphere is capable of learning to do what the other does and taking over for it. In fact, people can survive having a hemisphere of their brain removed. Particularly if it happens in childhood, when neuroplasticity is greatest, they can sometimes go on just as normal, with everything being done by just the one hemisphere.
Now suppose the scientist, instead of removing my whole brain, removes just my right hemisphere. He puts it in a new body, B2, which was cloned from B. At exactly the same time, his assistant removes my left hemisphere from B and places it in B3, also a B clone. Poor old B, now brainless, itself goes in the incinerator. The persons in B2 and B3 both wake up and go about their business, each believing that they are me. (If we like, we could even add that, before the surgery, the scientist brought it about that the two halves of my brain were duplicates of one another, so that the person in B2 is identical in their memories and personality capabilities to the one in B3, and to me before the surgery.)
Now the question is which, if either, is me–the person in B2 or the one in B3 or neither? Each person’s brain is partially physically continuous with my own. But it doesn’t make sense to say that each person is partly me, in the sense that each is partly the same subject of experience and partly not. (What would it be like for me to be partly the person in B2 and partly the one in B3?) Neither is it possible that they both be identical to me. At present each is going about their own business, having different experiences, so they aren’t identical to one another. Yet, by the transitivity of identity, if X=Z and Y=Z, then X=Y, so that if they are both identical to me, they must be identical to one another as well. 
So it looks like there are now three possible outcomes: I am the person in B2; I am the person in B3; I am neither person, but instead died as the result of the surgery, with two new people coming into existence rather than one. We can imagine situations that make which of these outcomes will obtain seem extremely important. For instance, maybe the scientist is going to release B2 but torture B3, and I want to know what will happen to me. And I presumably don’t want to die in any event.
But ask yourself: even with complete knowledge of all the relevant facts, how could I go about trying to answer the question of which individual (if any) I will be? What about the physical states in question could entail that one outcome obtains, rather than another? It seems clear to me that nothing about them does, or could. It seems metaphysically possible that, once the anesthesia wears off, I will be looking through the eyes of B2, or of B3, or of no one at all.
However, the relationship between the fundamental physical facts and such facts about the survival of composite material objects as there may be does not seem to be contingent in this way. For instance, suppose we think that my brain, and therefore I, cease to exist when the operation in question is performed. Presumably this will be because of some fact about the essence of my brain, such that it isn’t able to persist through that kind of change. It wouldn’t be possible to perform exactly the same operation, under exactly the same conditions, with exactly the same result, with the only difference being that my brain happened to survive this time. The same is true if we say that my brain survives as one hemisphere or the other, that it partially survives as both, etc. (If you tempted to say that whether “my brain,” as a separate object over and above the two hemispheres, survives the operation is a kind of pseudo-question, then good–hold that thought. It will be relevant to the arguments in the next post.)
If this is correct–which body I inhabit after the operation, if any, is a contingent matter, but whether any given material object survives the operation, and which subsequent object it is identical to, is not a contingent matter–then it follows that I must not be a material object. Substance dualism, however, immediately accounts for all the relevant facts. I am actually a non-physical soul, with the causal connections between myself and my body being contingent ones–presumably specified by some set of contingent psycho-physical laws. These laws could have any number of different forms producing any of the possible outcomes mentioned above, explaining both why any of these could happen and why it is impossible to determine a priori which actually will happen. So substance dualism accounts for the modal facts in a way that physicalism does not.
Further, and pertinently here, I think that this style of argument can avoid the worry I had about the modal arguments above. The worry there was that, though I could conceive of someone existing without their body, perhaps I couldn’t really do so, since, unbeknownst to me, I am essentially material. But it doesn’t seem to me that anyone physically like me could be such that it is essential to them that one result rather than another obtains in the transplant case: that seems far too arbitrary. I claim, then, that reflection on transplant cases like this provides a powerful argument for substance dualism. In the next post, I will discuss two more such arguments.
Really appreciate Dustin’s responses to the old “conceivability doesn’t entail metaphysical possibility” objection, I think my credence in dualism is steadily going up:)
I’ve never found modal arguments for substance dualism to be all that persuasive. My ability to conceive of existing without my body (or existing in a different body) depends on me conceiving of being an immaterial soul rather than being a purely physical being. So to use this conception as an argument that I *am* an immaterial soul is circular reasoning.
I think the point of the argument is more to show that it is possible we are immaterial souls. I presume that he goes on to give the evidence for why we actually are immaterial souls in the next post.