Phil 89: Applying Leibniz’s Law in Support of Dualism

Some materialists think that you and/or your mind or self is one individual object m, and your body (or maybe just your brain) is an individual object b, and that m and b are numerically identical. As we discussed before, materialists don’t have to think this. A materialist always has the option of denying that the mind is any kind of substance or object. They could say that minds are more like hikes or dances. What makes them a materialist is that they think all the objects there are, are physical. But they don’t have to say that the minds are some of those physical objects, anymore than we have to say that hikes or dances are.

But suppose you are the kind of materialist who does want to say that m is an object, and that it’s the same object as some physical b, such as your body or brain.

Dualists on the other hand think there’s a non-physical individual object s, namely your immaterial soul, that could in principle exist without your body. Many (but as we’ll see later, not all) dualists will say that m = s. All dualists will say that mb. That is, they’ll think that m and b are not numerically identical, where b is your body or brain or any physical object.

We were looking into how the dualist might argue that mb. Are there any properties that m and b seem to differ with respect to, so that dualists can appeal to Leibniz’s Law to show this?

Using Leibniz’s Law, Part 1

Dualists want to argue “minds can’t be physical because physical objects are … but minds are …”

The materialists who want to identify you and/or your mind or self with some physical object will resist these arguments. I’ll sort the dualist’s arguments into several different groups, differentiated by what kind of complaint the materialist will make about them.

Some of the dualist’s arguments may have the same problem as our Ronald Reagan example discussed earlier. Here’s an example:

  1. Physical objects (brains, bodies) all take up space.
  2. But minds are mental.
  3. So minds are not physical objects.

This argument can only succeed if being mental is incompatible with taking up space. And we haven’t yet settled that. That’s just what the dualist and the materialist are arguing about.

So the dualist’s arguments should always have the form of saying the one thing has some property, but the other thing lacks that same property.

Let’s try to fix up the previous argument to say that, explicitly.

  1. Physical objects (brains, bodies) all take up space.
  2. But minds do not take up space.
  3. So minds are not physical objects.

This is an improvement, in that it’s a clear application of Leibniz’s Law. It does seem right that if the premises of this argument are true, the conclusion would have to be true too. The problem though is that the second premise is too controversial to be helpful to use in an argument for dualism. It’s similar to if I were trying to persuade you that God existed, and I argued: “God exists because the Bible says so; and nothing written in the Bible is false.” It may be that everything I’m saying is correct, but you probably wouldn’t be very impressed or moved by my argument. The premise “nothing written in the Bible is false” is just too controversial to appeal to when what we’re trying to establish is whether God exists. Only someone who was already persuaded of the conclusion we’re aiming for is likely to be ready to accept the premise. This is the kind of argument that philosophers call question-begging.

Some arguments are really obviously question-begging, in that you can just see the conclusion among the premises: “God exists, and wrote the Bible. So God exists.” In other arguments, like the one I described a moment ago with the premise “Nothing written in the Bible is false,” the conclusion might not explicitly be among the premises. Still, these arguments are going to be unpersuasive because they start from points which are argumentatively too close to the conclusions they’re trying to establish. A good, persuasive argument should instead have more of a surprising punch to it. It should make its audience see that premises that you’d think are uncontroversial, or at least much less controversial than your conclusion, when taken together in fact do force us to accept that conclusion, after all.

The “Nothing written in the Bible is false” argument wouldn’t do that. And neither would the “Minds do not take up space” argument.

Here are other arguments that have the same feel:

  1. My brain weighs 5 pounds.
  2. My mind doesn’t weigh 5 pounds.
  3. So my mind is not identical to my brain.
  1. Minds are capable of thoughts and sensation.
  2. But physical objects aren’t capable of thoughts and sensation.
  3. So minds are not identical to any physical object.
  1. My brain has physical parts (as some philosophers put it, it “is divisible”).
  2. My mind doesn’t have any parts. (So we don’t need to argue about whether its parts are physical or not.)
  3. So my mind is not identical to my brain.

In each case, a materialist, or someone who was just trying to make up their mind whether to accept materialsm or dualism, wouldn’t be ready to accept the second premise. If the dualist wants to talk us into their view, they should try harder to find premises that seem plausible even to people who haven’t yet subscribed to dualism.

Using Leibniz’s Law, Part 2

Another group of arguments for dualism at first seem like they might achieve that. Consider:

  1. Aunt Lobelia is in my mind — I’m thinking of her right now.
  2. Aunt Lobelia is not in my brain — how could she fit? Go ahead and cut it open, I guarantee you won’t find her there.
  3. So my mind has a property — having Aunt Lobelia in it — that my brain lacks.
  4. So my mind is not identical to my brain.

Here premise 1 seems to be true, at least as it’s most naturally understood. And premise 2 also seems to be true. And the rest seems to be just an application of Leibniz’s Law. So is this a good argument for dualism?

The problem here, as we discussed in class, is that the sense of “in” where it seems to be clearly true that Aunt Lobelia is in my mind doesn’t seem to be the same sense of “in” where it seems to be clearly true that Aunt Lobelia is not in my brain. For the latter, let’s say “spatially inside.” Sure, Aunt Lobelia is not spatially inside my brain. But is she spatially inside my mind? Don’t think we want to say that. The sense in which she clearly seems to be “in” my mind is that I’m thinking about her, I have thoughts concerning her. It’s not at all clear that that means she is spatially inside my mind. (The dualist wouldn’t want to say she’s spatially inside my mind, anyway! Since they think minds don’t take up space.)

So this argument equivocates. Premise 1 seems plausible if we understand “in” with one meaning (“in my mind” = thinking about). Premise 2 seems plausible if we understand “in” with a second, different meaning (“in” = spatially inside). And if you don’t notice that “in” has changed meanings, it might look like the reasoning structure is correct. But for it really to be correct, we have to understand “in” consistently, with a single meaning.

The dualist may think that the argument is correct with “in” understood consistently in the sense of “thinking about her.” That is, they may accept all the premises and the conclusion of this argument:

  1. My mind is thinking about Aunt Lobelia.
  2. My brain can’t think about Aunt Lobelia, because it’s physical.
  3. So my mind isn’t identical to my brain.

But now we’d once again have an argument whose second premise is too controversial to be persuasive, in an inquiry about whether or not to be dualist.

Using Leibniz’s Law, Part 3

Where have we gotten so far? The materialist complains that if the dualist makes sure to be explicit about what property it is that the mind has but that physical objects lack (or what property it is that physical objects have but the mind lacks), the argument is going to be question-begging. Nobody is going to find the argument persuasive unless they’ve already subscribed to dualism.

The only times so far when it’s seemed otherwise, it was because the dualist was equivocating, and switching the meaning of some word halfway through the argument. One premise sounded plausible with the word understood one way, and the other premise sounded plausible with the word understood the second way.

But it turns out there’s a group of arguments for dualism that seem to be vulnerable to neither of these complaints. These are arguments where the premises do all seem to be plausible, even to people who haven’t yet subscribed to dualism. And it’s not obvious that any words need to switch their meaning halfway through the argument, for the premises to seem obvious in this way.

The general flavor of these arguments can be summarized like this:

  1. I have some special or privileged access to my own mind.
  2. I don’t have that kind of access to anyone else’s mind, nor to facts about my brain or body or physical environment. My access to my own mind is “better or different” than to those other things.
  3. So my own mind has a property — being accessible to me in this special way — that physical objects lack.
  4. So my mind is not identical to any physical object.

The specifics of these arguments will turn on how they unpack the notion of “special access.” In what sense is my access to my own mind “better or different”?

Here are some things philosophers say to explain what they mean here. (Many philosophers would say we have special access to our own minds in several of these senses.)

  1. I and only I can know my own mind’s properties without evidence, observation, or inference.

    Many of our mental states are ones we can know about in an especially direct way. You can just tell whether and when you’re thinking about elephants.

    On the other hand, other people have to infer what you’re thinking, from your behavior and what you say. So you have a way of knowing about your own mind that other people lack. And you don’t seem to have this access to their mind, either. Nor do you seem to have it to facts about bodies or brains or your physical environment. You can’t tell how much your brain weighs, or whether your body has paint on it, without looking at or touching or measuring them (or Googling it). You can’t tell what your shoe size or height are in this special direct way — at least not for the first time. Also, other people could in principle be in better positions to know these things than you are.

  2. This last point leads to a different idea, that our access to our own minds is better than other people’s access to it (and better than our access to their minds, and to our bodies and brains). Sometimes this is spelled out by saying we can’t make mistakes about our minds, or at any rate about some of our mental properties. If I think that I feel upset, it must be true that I feel upset. That’s not something I could be wrong about. And if I do feel upset, then I’ll know that I feel upset. But of course I could think that other people, or my body, has certain properties when really they don’t. Or they could have certain properties without me knowing that they do.

    It’s controversial how much of my own mind I really have this kind of can’t-make-mistakes access to. Arguably I can have some mental properties (such as being jealous of your lottery win) that I’m not aware of having, and believe that I don’t have. But if there’s even part of my mind that I have can’t-make-mistakes access to, that may make my mind different from any physical object. Presumably I could in principle make mistakes about any of my body or brain’s properties. Maybe I don’t even have a body or brain, but just seem to.

  3. This leads to a different idea, namely whether it’s possible to intelligibly doubt the existence of my body, versus my mind or self. The possibility that I don’t have any body or brain right now, that all my perceptions of a physical world are an illusion, is very unlikely to be correct. Still it’s a possibility I can make sense of. I can coherently imagine everything seeming the same to me — just the way it really seems — but that I in fact have no body or brain. Perhaps I’ve already died and this is all an illusion. Or perhaps I never really had a body in the first place. I can make sense of these alternatives. This is what I mean by saying “I can intelligibly doubt” that my body exists.

    Is it possible to intelligibly doubt the existence of my mind in the same way? That’s much more problematic. If everything seemed to me just the way it really does, wouldn’t I/my mind have to be there to experience that? If my mind can even ask the question whether it exists, it has to exist in order to do so. So it looks like I can’t intelligibly doubt that my own mind exists, in the way that I can have intelligible doubts about my body.

The details are controversial, but many philosophers would agree that our access to (at least some parts or aspects of) our own minds is “special” in some of these ways. And we don’t seem to have the same special access to anyone else’s mind, nor to facts about our brains or bodies or physical environments.

Perhaps this can be used in an argument by Leibniz’s Law that our minds aren’t identical to anything physical.

For example, appealing to “specialness” in sense (a) or (b):

  1. My mind is not “publically accessible”: there are ways to know about it that are in principle only available to me.
  2. Physical objects are all publically accessible: in principle, anybody could get into the best positions to know about them.
  3. So my mind isn’t a physical object.

Or appealing to “specialness” in sense (c):

  1. I can’t intelligibly doubt whether my mind exists right now.
  2. I can intelligibly doubt whether physical objects exist right now. Maybe I’m in some kind of Matrix, and everything seems real, but it’s all an illusion. That at least makes sense.
  3. So physical objects are such that I can intelligibly doubt their existence — I can at least imagine them not existing even though everything seems the same — but my mind lacks that property.
  4. So my mind can’t be identical to any physical object.

These applications of Leibniz’s Law are more challenging than the ones we considered before. It’s less clear how the materialist should respond here. What do you think?