What Does Metaphysical Dualism Say about Reality?

FROM THE LECTURE SERIES: Redefining Reality: The Intellectual Implications of Modern Science

By Steven Gimbel, Ph.D., Gettysburg College

If researchers succeed in the ultimate goal of physics, a single coherent theory that unifies the working of all four known forces in the universe, what does this mean for our picture of reality? Would a theory that explains how all matter, energy, and interactions among and between them, in terms of field values and symmetries, give us an exhaustive account of all we should consider to be real?

A hairy beast with two long, curved horns, hidden in dark and shadow, with the fires of hell burning behind it.
René Descartes’ evil demon thought experiment says that all ideas we have about the material world could be false as they were placed in our minds by an evil demon, but even if that were true for that to work the physical body must think, which means that humans are also a thinking thing, a thing that thinks—a mind. (Image: Poidl/Shutterstock)


This view hinges on what we’ve discussed as the doctrine called reductionism, that is, that everything studied in every science reduces ultimately to physics. According to this reductionist position, everything that exists is made up of nothing more than the elements of physics, there are no other extra-physical entities in the world. There is another key claim, but we will investigate that in more detail later in the course.

Learn more about defining reality.

Life is a Metaphysically Different Thing

Reductionists contend that everything in the universe is made up of atoms and energy. The longstanding resistance to this claim points to the existence of life as a metaphysically different kind of thing.

If we make a living being immediately before and then the instant after death, all of the atoms that were there beforehand are thereafter, but the thing itself is fundamentally different. We differentiate linguistically between a person and a corpse. We think intuitively that the two are different.

Another word we use for a corpse in ordinary language is the body. When we say somebody has died, we say that his or her body is here, but that he or she—that is, the person—is gone. We equate being a person with not just having a body, but also something else.

Learn more about the reality of atoms.

Metaphysical Dualism and René Descartes’ Evil Demon Thought Experiment

Traditionally, this something else is a soul. This two-part picture of reality, that there are two separate sorts of things, in reality, physical bodies and non-physical souls, is what philosophers call metaphysical dualism. The great name attached to dualism is René Descartes.

Talking about his evil demon thought experiment, Descartes said, “Consider the possibility that we are being fooled by an evil demon who’s putting false ideas in our minds”.

He’s not saying it’s true but argues that it’s conceivable, merely possible. And if it were true, then any given thing I think, any proposition I believe could be false, put into my brain by the demon. Hence, I could believe nothing about the material world, not even that it exists, since it all may be an illusion—the result of a complex hallucination created in my mind by the demon.

It seems like I could believe nothing about reality. But, he realizes, there is one thing even the evil demon could not wrongly convince me of—that I exist. The demon could not fool me into wrongly believing that I exist because I would have to exist to be fooled.

The demon would have to create the thought of existence but the thought requires a thinker, and thinking is doing and doing requires a doer. If I am thinking, I must exist; and to be fooled, I must first think. So, I can’t be wrong about my belief that I exist. But what am I?

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

I’m not the person I think I am with the body I think I have in the world I think I inhabit; all those beliefs could be the result of the demon’s handiwork. The only thing I have the right to believe with absolute certainty is that I am a thinking thing, a thing that thinks—a mind.

Learn more about the brain and self.

Mind is Separate From the Body

Portrait of René Descartes, with long hair, looking straight at the camera, wearing a garment with huge collars that hide   his neck from view.
René Descartes concluded that the mind exists separate from the body, but that gave rise to a major problem about the interaction between mind and body, even though they are completely different sorts of entities. (Image: After Frans Hals/Louvre Museum/Public domain)

And so, Descartes concludes, the mind exists separately from the body. He later goes on to try to prove with mathematical certainty that the body exists and that there are absolute laws of mechanics that the body has to obey; but these laws are the laws of matter, not those of the mind.

Mind and body are completely different things—different kinds of substances. Reality consists of two separate realms, the material world governed by the immutable laws of physics and the world of minds that are governed by logic, or psychological rules, or ethical rules, or no rules at all.

The human mind seems to be a different sort of thing endowed with a very important property—free will. We choose to do what we want. The problem for Descartes is that what we want is not just mental, but often physical. We want things, we want to do things. This means that these two separate realms—the world of matter and the world of mind—must interact with each other, this is problematic.

Learn more about the caring brain.

Interaction between Mind and Matter

On the one hand, we know that matter influences the mind. In the classroom, I’ll demonstrate this by walking upright to the big desk that’s in the front of the room, which is often laminated plywood top with metal sides, and I’ll kick the side of the desk hard; and the result is a huge reverberating, metallic bang.

I’ll then limp around the front of the room pretending to be injured. I ask, “What just happened?” And we see that there was a physical interaction, body-to-body when my toes hit the desk and rapidly contracted from the force. But this rapid contraction of my toes had a seemingly non-material effect; it gave rise to an idea, a mental experience. At first, I felt pain, then the thought, “Don’t kick solid things, you idiot”.

So, we have body-to-body interactions giving rise to a mental event, an idea, an experience.

In the other direction, we have a sense that the mind influences matter. What I’ll do then is I’ll ask one of my students to please write the word guacamole on the top of his page of class notes. When completed, I ask the student why the name of this zesty avocado dip is scrawled in his notebook and the response is usually, “Because you asked me to”

I then ask the student to stand up and flap his arms while clucking like a chicken.  Invariably, the student declines. When I ask why he says that he doesn’t want to.

So, the reason the word guacamole appears in the notes is not that I made you do it, but because you wanted to write it. Wanting, desiring, willing is a mental state, and somehow the idea in your head caused your hand to move in a very particular way that gave rise to exactly what you wanted to happen. We do not often go to write a word and have a completely different word show up. Our minds seem to control our hands.

The Dichotomy of Material and Mental Entities

So, it looks like we have two completely different things—material objects and ideas—and they are different sorts of entities. I can have a basketball and I can have the idea of a basketball, the two are not the same. One is material, the other is mental. One has size, shape, weight, location, the other doesn’t.

The basketball is orange and round, the idea of basketball is the idea of a round, orange thing, but it is not an orange idea or around the idea. The properties of material and mental entities seem completely different. Yet, the material seems to give rise to the mind through perception, and the mind appears to give rise to the physical through acts of will.

We seem to have interaction between them. But how? It’s the size, shape, and mass of one object that lets it act on another. If ideas have no physical properties, how can they give rise to physical movements like the hand moving the pen across the paper? This problem of interaction seems to plague the move to include non-material things in the universe, to add a completely new category of stuff to reality.

Common Questions about Metaphysical Dualism

Q: What are the examples of metaphysical dualism?

One of the best examples of metaphysical dualism is body and mind. These are two completely different things that makeup reality, but there’s no concrete answer for how the two interact with each other.

Q: What is the theory of dualism?

The theory of dualism or metaphysical dualism contends that the true picture of reality has two parts – physical bodies and non-physical minds. It’s separate from the reductionist view that everything in the universe is made up of atoms and energy, and nothing else.

Q: Who believed in dualism?

French philosopher, René Descartes championed the theory of dualism. His evil demon thought experiment illustrated that human beings were more than just a physical body. The evil demon – an external entity – can plant thoughts inside this physical body, but for that, to work the physical body must think. So, humans are also a thinking thing, a thing that thinks—a mind.

Q: Why is dualism wrong?

There is a fundamental problem with dualism. We know that matter influences the mind, and we also have a sense that the mind influences matter. So, what we have are two completely different things—material objects and ideas—interacting with each other. But if ideas have no physical properties, how can they give rise to physical movements? This problem of interaction plagues the theory of dualism.

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Gradual Redefining of Matter and Reality: From Mechanics to Quantum
The Three Pictures of Reality
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