Saving Descartes’ Theory of Mind: Substance Dualism to Property Dualism

Ryan Hubbard, PhD
Mar 26, 2020 · 8 min read
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Photo by David Matos

Mindfulness meditation is becoming increasingly popular in the west. There’s a good reason for this since it can make us happier, relieve stress and help us focus. Mindfulness meditation often involves simply sitting and maintaining an awareness of your breathing. After a while, you gain a heightened awareness of the content of your inner world: thoughts, beliefs, emotions, moods, sensations, the feeling of embodiment, and everything else. The philosophy of mind examines this content and, perhaps more importantly, the ‘container’ of this content: the mind

Even though it’s difficult to pin down exactly what a mind is, we all have them. We also have bodies composed of bones, organs, muscle, a nervous system, and so on. This is all material, physical stuff. But our minds aren’t material stuff. We can observe that someone feels pain by looking at his facial expression or brain activity. We can observe his behavior. However, we can’t experience his pain. We cannot observe it from his point of view as we can material things. Regarding his pain, all we can observe is his behavior and neural activity. We can only observe minds from the first person. This makes them extremely unique things.

Science fiction often explores the relationship between mind and body. Ghost in the Shell, for example, is about a person whose consciousness is transferred to a cyborg body. The movie Transcendence is about a scientist who uploads his consciousness into a computer. Many of the episodes of Black Mirror play with the idea of transferring a person’s mind to another entity.

The premise behind these stories is that one’s mind can be separated from one’s body. This idea seems to come naturally to us and lends itself to a view called mind-body dualism. Whether this view is correct has interesting implications. For example, if this view is true, then perhaps our minds do not depend on a particular substrate and can be transferred from a carbon-based to a silicon-based substrate. If mind transference is possible in theory, then mind-body dualism is an accurate philosophy of mind. René Descartes espoused this view.

According to mind-body dualism, the mind and body are distinct in some kind of way. Different dualistic views will give different accounts of how the mind is distinct from the body. René Descartes’ brand of mind-body dualism is called substance dualism since he believed that the mind and body are different substances: mental and material.

Substance dualism isn’t popular among philosophers today, but it’s important to examine because it’s part of a long pedigree of theories of mind and helps us grasp contemporary accounts of consciousness. For example, one contemporary dualistic account of mind is property dualism. This view claims that the mind exists as a set of properties that are fundamentally distinct from material properties. It differs in important respects from Descartes’ view, but his arguments contain important insights for these contemporary accounts of mind. In fact, I think Descartes’ arguments for substance dualism actually end up supporting property dualism.

No matter what content of the mind is removed, there is still a single, unified mind that is experiencing whatever content is left.

I’ll start with two arguments Descartes gives in his Meditations on First Philosophy to support Substance Dualism.

I’ll call Descartes’ first argument the Argument from Conception since it hinges on how we conceive of mind and body. Here’s the bare-bones version:

1. I have a clear and distinct idea of myself as an unextended, thinking thing.

2. I have a clear and distinct idea of my body as an extended, unthinking thing.

3. Therefore, “I” (my mind) must be distinct from my body.

This first thing to notice is that Descartes is identifying his self with his mind. This has interesting implications for Descartes’ understanding of the nature of the self. For our purposes, we’ll use “self” and “mind” interchangeably. We should also note that this argument is undertaken from a first-person perspective. Working through it, then, requires reflecting on our own conceptions from the first-person point of view.

The first premise states that I have a conception of my self — my mind — as unextended and thinking. When I reflect upon my own mind, I conceive of it as thinking. After all, the very process of conceiving is itself a form of thinking. For Descartes, when we conceive of our minds, we also find that it is unextended. In other words, it is not something that takes up space. When we conceive of our body, we find that it is extended, that it takes up space.

The key to Descartes’ Argument from Conception is implicit and conceptual. The crucial, implicit premise is that if I can conceive of two things with distinct, incompatible properties, then those two things must be metaphysically distinct, that is, they must exist as distinct things in external reality.

Here’s an example: I can conceive of something (x) that weighs well over 100 pounds and something (y) that weighs well under 100 pounds. Something cannot have the property of being both over and under 100 pounds. So x and y must be distinct. Here’s another example. An animal cannot be both a mammal and a reptile. Being a mammal and being a reptile are incompatible characteristics. Therefore, it’s necessarily the case that mammals and reptiles are distinct.

Descartes has given a powerful argument that the mind and body are distinct…His core insight is that the mind and body have incompatible properties, so they must be distinct.

To look back at Descartes' argument, unextended and extended are incompatible properties: something cannot both extend and not extend in space. Since the mind is essentially unextended and the body is essentially extended, then they necessarily cannot be the same thing. Therefore, the mind must be distinct from the body.

Descartes' next argument for mind-body dualism is similar to the first. However, with this argument, which we’ll call the Argument from Divisibility, Descartes isn’t appealing to how we conceive of the mind and body. Rather he is addressing the metaphysical nature of the mind and body directly. As we’ll see, Descartes uses the same argumentative strategy for both arguments. Here’s his Argument from Divisibility.

1. The mind is indivisible.

2. The body is divisible.

3. Something cannot be both divisible and indivisible.

4. Therefore, the mind and body must be distinct things.

Let’s start with the second premise. Descartes is saying that the body can be divided. This is obviously true. The third premise is also true since divisibility and indivisibility are incompatible properties. This premise is analogous to saying that something cannot be both extended and unextended. So, the argument hinges on the first premise. Is it true that the mind cannot be divided?

There seem to be many parts of the mind. The stream of our conscious experience contains physical sensations, feelings, thoughts, and so on. Therefore, it may seem like we could divide the mind by taking away one of these parts. It’s possible, for example, to remove the part of the brain that enables us to experience certain physical sensations thereby dividing and removing that part of the mind. However, Descartes could respond by saying that doing this may alter the mind, but it’s not dividing it. No matter what content of the mind is removed, there is still a single, unified mind that is experiencing whatever content is left.

Perhaps a psychiatric condition such as multiple personality disorder is an example of a divided mind. This may point us toward showing how the first premise is mistaken. To defend Descartes, however, we could argue that in such psychiatric cases the mind isn’t divided, rather, there simply exist multiple minds connected to a single body.

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At this point, Descartes has given a powerful argument that the mind and body are distinct in some way. His core insight is that the mind and body have incompatible properties, so they must be distinct. But he then makes a hasty leap to the conclusion that they must be distinct substances. This is problematic since substance dualism has a difficult time explaining how the mind and body interact.

Descartes believed that the mind and body affect one another. It’s uncontroversial that the body affects the mind. If I take a bite of Key lime pie my taste buds send a signal to my brain, which releases endorphins. This causes me to experience pleasure and so has an effect on my mind. Much of what happens in my body alters my mental experiences.

Even though Descartes’ substance dualism is problematic, we can still retain his core insight.

It’s more controversial, however, whether the mind can have a causal effect on my body. Descartes thought that the mind can affect the body, but most contemporary dualists disagree. One kind of contemporary dualism called epiphenomenalism, claims that the body affects the mind but not vice versa. The mind is more like a byproduct of the body and brain, much like a shadow is to a tree that casts it. The tree causes the shadow to appear, but the shadow has no effect on the tree.

The problem with Descartes’ brand of dualism is that it’s very difficult to explain how two fundamentally different substances could casually interact. Plato’s two-world account of reality faced a similar problem.

How can material and immaterial things affect one another? This seems to defy the laws of physics. At the least, the laws of physics are not equipped to explain this causal interaction, since such laws concern only physical things. Descartes thought that the mind and body interact through the pineal gland. This explanation, is, of course, simply mistaken.

Even though Descartes’ substance dualism is problematic, we can still retain his core insight. We don’t have to throw the baby out with the bathwater. The baby, in this case, is Descartes’ idea that the mind and body have incompatible properties and so the mind must be distinct from the body, at least in some manner.

To save his insight, we can — after a lot of work — infer that the mind is a distinct set of properties rather than a distinct substance. The picture we get is one set of mental properties and another, distinct set of physical properties. Both sets of properties supervene upon a single material substance that composes the body and brain. The result: property-dualism.

A Philosopher’s Stone

A place for a discussion of the ideas all around us in…

Ryan Hubbard, PhD

Written by

A philosophy professor who works in practical ethics. @ryankhubbard

A Philosopher’s Stone

A place for a discussion of the ideas all around us in society, culture, philosophy, and more.

Ryan Hubbard, PhD

Written by

A philosophy professor who works in practical ethics. @ryankhubbard

A Philosopher’s Stone

A place for a discussion of the ideas all around us in society, culture, philosophy, and more.

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