The Mind Body Problem

© 2011 By Paul Herrick

The Mind-Body Problem

You have a mind and you have a brain. What is the relationship between the mind and the brain? Is the brain one thing while the mind is another thing entirely? Are mind and brain two distinct things? Or are mind and brain one and the same thing? Is mind identical to brain? Is the functioning of the mind simply nothing more than the functioning of the brain? If the brain is not the same thing as the mind, then how are they connected or related? The brain is a physical entity composed of billions of cells called neurons, each ultimately composed of billions of atoms and molecules. If the mind is not the same thing as the brain, then how do they communicate? What is the mind made of, if it is not a physical (material) thing?

Here is another way to pose these questions: Our thoughts take place within a medium of some sort. Within what medium? Within the brain? Or within a nonphysical medium? What would a nonphysical or immaterial medium be like? Would it be what some call the soul? Is each person actually an immaterial soul or spirit operating a physical body, as most religions teach? Or is the physical body all there is to you? These are among the questions that make up the philosophical issue known as the “mind-body problem.” 

Mind-body Dualism

The French philosopher Rene Descartes (1596-1650) argued for a view that is known today as “mind-body dualism.” A view is dualistic if it maintains that two fundamentally different kinds of things exist; a view is monistic if it posits that only one kind of thing exists. Descartes argued that mind and matter are two radically different types of things. Our conscious mind is one thing, argued Descartes, and our physical brain, which is made of matter, is another thing entirely. How did he reach this conclusion?

Descartes lived at a time when people were questioning many traditional beliefs. Setting off in search of something that could be known with absolute certainty, Descartes began his philosophy by systematically and deliberately doubting everything it is rationally possible to doubt. His plan was to see how far this would go. If we carry the process of systematic doubt as far as it can go, he reasoned, perhaps we will eventually reach beliefs that cannot be rationally doubted. If we do, then we will have reached something we can know with complete and absolute certainty. Among the propositions he eventually claimed to have proven with certainty were two pertaining to the mind-body problem:

  1. The essence of matter is nothing but to be extended in space, that is, to occupy a volume of space.
  2. The essence of mind is nothing but the activity of thinking.
  3. From this he concluded:
  4. Since matter and mind have differing essences, the mind is not the brain, for the brain, being made of matter, is a purely material entity, and mind is not material in nature.
  5. Thus, the brain must be one thing and the mind must be another thing entirely. <START>
  6. The mind is therefore a nonmaterial or nonphysical entity.

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Arguing for Materialism

Opposed to mind-body dualism is materialism, the view that nothing exists but matter and things made of matter. (Matter may be defined as that which physics studies, namely, subatomic particles, atoms, molecules, fields, and things made of such things.) If materialism is true, then the mind cannot be a separate, immaterial entity existing apart from the physical brain. If mind is a material thing, as materialism proposes, then it is reasonable to identify the mind with the brain or with the functioning of the brain, since the brain is a material or physical thing and it is certainly associated with thinking. Materialism is a monistic view since it says all things belong to one fundamental kind of thing: matter.

The British philosopher Thomas Hobbes (1588-1679) was the first modern philosopher to argue systematically for materialism as a solution to the mind-body problem. Hobbes argued that everything in the world, including our thoughts and the states of our minds, can in principle be explained in terms of one thing and one thing only: matter in motion. Thoughts, for example, can be explained, he argued, as matter in motion in the brain. A perception of the world arises in a person’s brain when motion in the external world causes motion in the brain, which is then experienced as an external object, and so on. If everything can be explained as matter in motion, including the mental, then there is no good reason to suppose mind and matter are two fundamentally different kinds of things. Rather, it is more reasonable to suppose that if everything can be explained in material terms then everything is material. So argued Hobbes.

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Dualistic Arguments

Arguments for a dualistic solution to the mind-body problem generally employ the logical notion of identity and a logical principle known as Leibniz’s Law (named after the great German philosopher Gottfried Leibniz (1646-1716) who formulated a noted version of it).

Identity. When we say that A is “identical with” B we mean that A is the same entity as B, that is, A and B are one and the same entity. For example, Bob Dylan is identical with Robert Zimmerman, meaning they are one and the same person, not two different persons.

Leibniz's Law. For any x and any y, if x is identical with y, then any characteristic or property possessed by x is possessed by y, and any characteristic or property possessed by y is possessed by x.

This implies:

If x has a characteristic that y does not have or y has a characteristic that x does not have, then x is not identical with y.

The general format followed by arguments for dualism can be put as follows:

  1. The argument begins with the claim that our mental states have a characteristic or property that is not possessed by our (physical) brain states.
  2. An appeal is made to Leibniz's Law.
  3. It follows that our mental states are not identical with our brain states, i.e., our mental states are not the same as our brain states.
  4. Therefore, the argument concludes, our minds are not identical with our (physical) brains, i.e., our minds and our brains are distinct and separate entities.

The mental image argument and the intentionality argument are examples of arguments for dualism that fit this form.

The Mental Image Argument for Dualism

  1. When I form the image in my mind of a red stop sign, my mental image is red. But no part of my physical brain turns red when I form the image in my mind.
  2. So, my mental state has a property—the property of redness—that my brain state lacks.
  3. If x has a property that y lacks, then x must not be identical with y.
  4. Therefore, my mental state is not identical with my brain state, i.e., my mental state and my brain state are two distinct entities.
  5. Since my mental state is a state of my mind and my brain state is a state of my brain, it follows that my mind has a property that my brain lacks, and so my mind is not identical with my brain, i.e., my mind and my brain are two different entities.

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The Intentionality Argument

Mental states such as beliefs, hopes, desires, fears, and wishes possess an interesting property: the property or characteristic known in philosophy as “intentionality.” Essentially, intentionality is the property of aboutness. A state has intentionality if it is about something. A hope is always a hope for something. A person doesn't just believe or know, a person believes or knows something.

However, intentionality is not a property of matter recognized and measured in physics textbooks, and furthermore it seems that no purely material thing can be inherently intentional.

These considerations give rise to the intentionality argument which can be summarized as follows:

  1. Certain types of mental states, such as hoping, believing, knowing, wishing, dreaming, etc., are intrinsically intentional.
  2. However, no physical, purely material thing is intrinsically intentional.
  3. Therefore, by Leibniz’s Law, mental states such as hopes, wishes, desires, knowledge, etc., are nonphysical, immaterial in nature.
  4. But the brain is purely physical in nature—it is a physical or material entity.
  5. Therefore, mind and brain must be two different things.

Materialist Objections to the Intentionality Argument

Some materialists have argued that the following entity is a counterexample to the dualist claim that no physical object can be intentional: a book. After all, a book is a purely physical object, they argue, and yet a book is about something. So it is purely physical and also intentional at the same time.


In reply, dualists argue that although a physical book is intentional, it is not intrinsically intentional—its intentionality is not part of its inherent constitution. Rather, they argue, its intentionality is read into it by a human writer or reader. Thus, the human reader or writer is the source of the book’s intentionality. Its intentionality is thus “derived,” or “external,” not intrinsic, unlike the intentionality of a mental state such as a hope or a wish. This is why the dualist is careful to argue that no purely physical object can be intrinsically intentional. A book therefore is not a counterexample to the dualist claim.

According to the intentionality argument, intentionality is not a physical phenomenon. A materialist could rebut this claim by showing that intentionality is, after all, nothing more than a physical phenomenon. This could be accomplished by explaining how a purely physical process or thing could be intrinsically intentional. This could also be accomplished if intentionality could be broken down without remainder into purely physical components. From here, the materialist would have to show how the physical brain might possess intrinsic intentionality, since the materialist thinks the brain is the mind and since the mind obviously does have intentionality. Dualists argue that such a reduction will never be made and are not holding their breath.

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The Knowledge Argument

The knowledge argument is another argument for the claim that consciousness is nonphysical or immaterial in nature. Consider the following thought-experiment. A future neuroscientist named Mary, born and raised entirely in a black and white room, could in principle know all the physical truths about the brain. This is at least logically possible, for there is no contradiction in the idea. However, although this future neuroscientist would know all the physical facts about the brain and how it works, she would not know facts about consciousness, including what it is like to see red. For when she finally leaves the room and sees colored things, she learns something. (She learns what it is like to see red, something that cannot be fully translated into words or equations, argues the dualist.) Thus, there are truths about consciousness that cannot be logically derived from purely physical truths, that is, from truths about physical things. For instance, truths about what it is like to see red. Thus, consciousness is nonphysical or immaterial.

A Reply

In reply, some philosophers have argued that the neuroscientist does not gain any new factual knowledge, all she learns is a new ability—an ability, moreover, that can be fully explained in purely behavioral terms. But this ability only involves physical patterns of motion, not any knowledge of new facts. Therefore, the knowledge argument fails.

In response, defenders of the knowledge argument have argued that a new ability doesn’t capture the deeper difference in this case, namely, the difference between the black and white world and the world of color experiences, for this is a difference existing at the level of experience, namely, the experience of what it is like to see red. And such an experience is more than a mere ability. So argues the dualist.

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Modal Arguments

A number of arguments employing the modal notions of possibility and necessity have taken center stage in philosophy of mind in recent years.

The Zombie Argument

A zombie would be a creature physically identical to a conscious human being, acting in identical ways to a conscious human being, but that is not conscious. Zombies are conceivable, for there is no contradiction in the idea. Therefore, Zombies are logically possible. If Zombies are logically possible, then materialism is false and consciousness is not a purely material thing. Therefore, materialism is false.

Kripke’s Argument for Mind-Body Dualism

True identities, such as that heat is molecular motion or that lightning is an electrical discharge, are necessarily true, true in all possible circumstances, in other words, true in all possible worlds. If it is true that X is identical with Y in one world, this identity is true in all possible worlds. Given any mental state M (for example, a pain or an itch), and any physical state P (for instance, a group of neurons firing in the brain), we can imagine the mental state occurring in the absence of the physical state. Therefore, given any mental state and any physical state, it is possible the mental state exists without the physical state. Therefore, no mental state is identical to a physical state. Therefore, the mind is nonphysical.

In Reply

In reply to both of these arguments some philosophers have argued that the mental states in question, states that the dualist claims must be immaterial, can be explained in purely material terms, thus showing they are nothing more than purely physical states of organisms. Of course, in reply, defenders of dualism argue that the reduction of the mental to the physical fails because it fails to explain key aspects of the mental, namely, the first-person, subjective, experiential aspects. As you might imagine, the back and forth discussion gets extremely technical, which is why this is as far as we can take the issue in this short Interlude.

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General Objections to Dualism

The Causal Interaction Problem

This is perhaps the main philosophical objection to Dualism. The dualist claims that the mind and the brain are two distinct entities. Nevertheless, we all know that mind and brain at least interact with each other. That is, goings on in the mind can cause changes in the brain and vice versa. Therefore, if dualism is true, then the physical body sends signals to the brain and then to the mind, and the mind sends signals and commands through the brain to the body. (Mind-body dualism is thus also called “Interactionism” or “Interactionist Dualism,” since it posits that the mind and the body causally interact.) 

However, if dualism is correct, the mind is not located in physical space, but the brain is located in physical space. How could something that is not located in physical space cause or affect something that is located in physical space? It would seem that if A causes B, then A must be located somewhere in space and B must be located somewhere in space; indeed, A and B must be physically next to each other in some way. At least this is what everyday experience teaches.

Furthermore, according to dualism, the mind possesses neither mass, shape, nor momentum. Yet the brain is a physical object. Dualism maintains that mind and brain affect each other, but how could a massless, non-physical mind that possesses no momentum or solidity have any kind of effect on a physical object such as a brain? Such interaction wouldn't seem to make any sense at all. This causal connection certainly cannot be explained in scientific terms, for science deals only with material entities, events and processes.

Here's another way to put this point. If the dualist is right, the mind is a ghostly, non-physical entity that has no solidity or materiality. But the brain is solid, physical matter. How could a ghostly non-solid, nonphysical, invisible entity affect or cause a change in a solid physical entity?

A Dualist Reply

In every system of cause and effect, there must exist at least one cause and effect connection that is unanalyzable, or “primitive,” for analysis cannot go on forever. An unanalyzable causal connection would be a cause and effect connection that cannot be broken down into smaller parts. In response to the above objection, the dualist may theorize that the causal connection between the mind and the brain is simply one of these unanalyzable, primitive causal connections. If so, then it is simply a basic, unanalyzable fact about us that our immaterial minds have this admittedly mysterious causal power (the power to affect our physical brains and to be affected by the brain as well). In other words, perhaps the causal connection between mind and brain cannot be analyzed into a set of simpler elements or mechanisms and therefore cannot be made part of a scientific theory. Says the dualist: So much the worse for science.

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The Simplicity Argument

Cartesian dualism maintains that mental states are states of an immaterial substance, the mind, which is supposedly distinct from the brain. However, neurophysiology reveals that mental events correlate with physical events in the brain (or central nervous system). That is, when mental events occur, corresponding neural events in the brain (or central nervous system) occur at the same time. Dualism offers an explanation: the correlations reflect the interaction of two radically different things: mind and brain. On this view, the correlation between mental events and neurological events in the brain is like the correlation between the flipping of the light switch and the light coming on. In contrast, materialism offers a simpler explanation: mental events simply are neurological events in the brain (or central nervous system). In other words, mental events are identical with neurological events in the brain. Mind and matter are not two different things; minds are material things. This version of materialism is known as the “identity theory of mind.”

Now, the argument continues, the identity theory is preferable to dualism for one very important reason: According to the principle known as Ockam's Razor, if two hypotheses account for the same phenomena but one hypothesis is simpler, the simpler hypothesis is preferable. (Roughly, one hypothesis is simpler than another if it postulates fewer explanatory entities or fewer types of entities.) We should not posit additional explanatory entities in addition to matter (such as immaterial minds) unless they are needed to explain the phenomenon in question. The identity theory is explanatorily simpler than dualism, for dualism explains a human being in terms of two radically different substances—mind and matter—while the identity theory explains humanity in terms of one fundamental substance—matter. The identity theory thus accounts for mental phenomena in a simpler way, in terms of the brain alone, without adding an immaterial mind to the picture. Says the materialist: The dualist adds an additional but unnecessary element to the picture: a nonmaterial mind. On the basis of Ockam’s Razor, the identity theory is therefore preferable to dualism. J. J. C. Smart, one of materialism's prominent defenders, wrote:

Why do I wish to resist [dualism]? Mainly because of Ockam’s razor.

A Dualist Reply

If two hypotheses equally explain the data, the simpler of the two is preferable. The dualist admits that the identity theory is simpler than dualism. However, says the dualist, it must be remembered that Ockam's razor does not recommend accepting the simpler hypothesis when the simpler hypothesis can't explain important things that are explained by the more complex hypothesis. If there are phenomena that the simpler hypothesis cannot account for, while these things are explained on the more complex hypothesis, then in this case, the two hypotheses do not equally explain the data. In which case, even though one hypothesis is simpler than the other, the more complex hypothesis is preferable. In such a case, the simpler hypothesis is too simple.

The identity theory, argues the dualist, is too simple, for it cannot explain certain features of the mind. For instance, dualists argue, no materialist theory can explain the experienced mental states called “qualia.” Examples of qualia include the experienced color of a mental image, the experienced taste of chocolate, the experienced smell of a rose, and so on. In addition, argues the dualist, the identity theory cannot account for the “aboutness” of thought; that is, it cannot account for the intentionality of certain mental states. Dualism, on the other hand, can account for them. Or so argues the dualist. If these dualist claims are correct, dualism explains things that the identity theory cannot explain, in which case Ockam's razor does not require that we adopt materialism, the simpler theory.

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