Mind Brain Identity Theory

Introducing

The theory that we will examine in the present chapter—the mind–brain identity theory (aka “type-identity theory,” “psychoneural reductionism,” “central-state materialism,” and often simply “the identity theory”)—can be stated simply. It is the view that the mind is the brain and that mental states are brain states. Mind and brain are one and the same—they are identical.

Since the brain is physical, mind–brain identity theory is a kind of physicalism. It denies both substance dualism and property dualism. It denies substance dualism since it denies that the mind is a nonphysical thing. It affirms that the mind is a thing, but it is a physical thing—it is the brain. Mind–brain identity theory denies property dualism because it denies that mental properties, such as qualia, are nonphysical properties. They are indeed properties, according to this view, but they are one and the same as certain brain properties.

It is worth here noting a key difference between mind–brain identity theory and behaviorism, the other physicalist theory that we've examined so far. The main difference might be put like this: Where the behaviorist defines mental states directly in terms of outward behavior, the mind–brain identity theorist defines mental states as something literally inner, since a person's brain is something literally inside of their body.

Advantages of Mind–Brain Identity Theory

Let us briefly recount some of the problems that arose for the theories we've discussed so far, and reflect briefly on how mind– brain identity theory better solves them. We can summarize the main problems for the previous theories as follows:

• Substance and property dualism lead to epiphenomenalism and serious problems concerning mind–body interaction. Further, they make exceptions to the scientific worldview whereby everything is bound by the laws of physics.
• Idealism and the related theories of solipsism and panpsychism are deeply contrary to common sense.
• Behaviorism doesn't preserve the commonsense idea that many of our mental states are causes of behaviors.

The contrast between dualism and the identity theory is especially worth remarking on. Besides the general worry that dualism is not really consistent with a generally scientific worldview, a worldview that is overwhelmingly physicalistic, there is also the worry that arises having to do with the problem of interaction. The problem of interaction is especially acute for Cartesian substance dualism, which holds that minds are so radically different from physical bodies that they do not even have spatial locations. How, then, can a substance that is nowhere at all have causal effects on my body (as when I intentionally raise my hand)? A closely related problem is the problem of explaining why it is that my mind has a direct effect only on my own body. If your mind and my mind are equally nowhere, then why is it that my thoughts can have a direct influence on my body but not yours? And why does damage to your body cause suffering to arise in your mind but not mine?

The identity theory has a straightforward way of dealing with all of these questions and problems. Question: How can the mind affect the body and the body affect the mind? Answer: In the general way that physical objects affect each other, since the brain is as much a physical object as the rest of your body. Question: Why is it that damage to my body causes pain in me and not you? And why can I intentionally raise my own hand, but cannot directly raise your hand intentionally? Answer: Your brain is directly connected to the rest of your body, but not to my body, whereas my brain is connected to my body and not yours. Relatedly, identity theory does not lead to the troubling thesis of epiphenomenalism as property dualism does. Qualia are causally efficacious properties because they are brain properties, which are causally efficacious generally.

Let us now consider comparisons between identity theory and the mind-centered views of idealism, solipsism, and panpsychism. Unlike idealism and solipsism, identity theory takes a more commonsensical approach to existence. There are many physical objects that exist independently of our minds because our minds are just our brains and there are many physical objects that exist besides brains. Unlike panpsychism, which draws no strict division between physical systems that have minds and physical systems that do not, identity theory is able to draw a strict division. Physical systems lacking brains thereby lack minds.

Aside from the virtues of identity theory already outlined—virtues such as explaining mind–body interaction in a satisfying way and avoiding epiphenomenalism—an argument sometimes given in favor of identity theory is that it is simpler than its main competitor, dualism. As discussed in chapter 4, the idea that the simplest of competing theories is the preferable one is known as Occam's razor. Occam's razor arguably favors identity theory over dualism. Whereas dualism postulates entities in addition to brains and mental properties in addition to brain properties, mind–brain identity theory is simpler for dealing with just brains and brain properties. If dualism is identity theory's only competitor, then identity theory wins the competition for simplest theory.

However, we shouldn't be too quick in concluding that Occam's razor settles all debates in favor of identity theory. As we know from chapter 4, there are other competitors besides dualism—there are the varieties of idealism, such as solipsism. Some of these are very simple indeed. If only your own mind exists, that would indeed be very simple! This is not to say that your mind is simple. The point here is that your mind plus an external world is more complex than your mind alone. And the larger point is that the question of which theory is simplest is itself not so simple! 6.9 Before moving deeper into the complexities of the philosophical debates over mind–brain identity theory, it will be useful to briefly review some of the science relevant to the mind and brain.

A Very Brief Overview of Neuroscience

Neuroscience is the scientific study of the brain and other parts of the nervous system of humans and other animals. The brain, especially the brain of humans, is one of the most complex things in the universe. Accordingly, neuroscience is a complex field of study. Nonetheless, we can briefly say some things about the main features of our scientific understanding of nervous systems.

Major parts and functions of the nervous system 6.11 The nervous systems of animals (humans included) serve as the major means via which information is relayed, processed, and stored within their bodies. Vertebrate animals, including humans, have two main portions to their nervous systems—the central nervous system and the peripheral nervous system. The central nervous system includes the spinal cord and the brain and thus is the portion of the nervous system most directly involved in cognition and consciousness. The peripheral nervous system relays signals to and from the central nervous system. It relays signals from sensory organs and signals to muscular systems. In vertebrates, the brain is located in the skull and close to the major sensory organs, especially those for vision, hearing, taste, smell, and sense of balance.

Major parts and functions of the brain

Of the major portions of the brain, the portion that most distinguishes humans from other vertebrates is the cerebral cortex (often just called “the cortex”). The cortex is especially complex and large in humans compared to other animals, with the most significant differences having to do with the frontal cortex (the front part of the cortex). The cortex forms the wrinkled outer surface of the brain. It is highly wrinkled in humans compared to other animals because of how much more of it must be crammed into the comparatively small space of the skull. (Think of how very wrinkled clothes can get when a lot of them are crammed into a very small piece of luggage!) Various cognitive functions can be somewhat localized in distinct regions of the cortex. However, localization studies comprise an area of ongoing research and much remains to be discovered about which regions of the cortex perform which cognitive functions and how. Nonetheless, there is widespread agreement about the following rough assignment of functions: The posterior (back) half of the cortex is largely dedicated to sensory processing, with a very large portion of that (the backmost portion) dedicated to vision. The frontal half of the cortex is largely dedicated to motor processing (processing that eventuates in muscular movements) and executive functions such as the planning and control of voluntary behaviors.

Neurons, neural activations, and brain states 

Nervous systems are composed primarily of two sorts of cells—neurons, which relay electrochemical signals to one another, and glia (also called “glial cells”), which are largely dedicated to supporting the functioning of neurons. Protruding from the body of each neuron is a long axon and many, profusely branching, dendrites. A synapse is the site of connection between a neuron and another cell via which the neuron relays a signal to the target cell. Electrical and chemical signals move both within neurons and between them at the synapses. Short-lasting electrical events, known as “action potentials,”“nerve impulses,” or “spikes” play a central role in neural signaling. When a neuron emits an action potential, it is said to “fire.” There are many kinds of specialized neurons. One kind is the photoreceptor neurons in the eye that transduce light into electrochemical neural signals. Another kind is the somatic neurons that connect directly to skeletal muscles and are crucial for bodily movement.

Lesions, imaging, and electrophysiology

Neuroscientists have developed many techniques for studying the functioning of living brains. One class of techniques involves observing spared and impaired functions associated with damage (lesions) to regions of the brain, including both accidentally caused and deliberately caused lesions. Other techniques involve machines that can create images of brain regions and the activity within them. One such technique, known as functional magnetic resonance imaging (fMRI) detects changes in blood flow associated with changes in neural activity. Yet another class of techniques measures electrical activity in various brain regions, either by recordings from electrodes on the scalp or by small sensors inserted directly into individual neurons.

Localism and holism

One general line of questioning in neuroscience concerns the degree to which a cognitive function is performed by a specific part of the brain that is specialized for just that one function. The view known as localism is the view that cognitive functions are localizable in brain regions dedicated to performing those functions. So, for example, there would be a specific brain region for language, a different region for memory, and still a different one for vision. The view known as holism opposes localism. In its most extreme form, holism is the view that the entire brain subserves each cognitive function and that any particular part of the brain plays a role in every cognitive function. For any particular function, such as the visual perception of color, controversy surrounds the question of how localized (versus distributed) a function is in the brain. An early and extreme version of holism held that damage to a part of the brain may result in an overall degradation in cognitive functioning, but would not completely wipe out just a single function. More recent lines of evidence point away from this extreme holism. For instance, damage to a specific part of the brain can destroy a person's ability to consciously perceive the shapes of objects while leaving intact other aspects of vision as well as nonvisual cognition.

Some General Remarks about Identity

Before getting deeper into mind–brain identity theory, we are going to pause here to discuss the general notion of “identity.” What does it mean to talk, in a general way, of things being identical?

The core notion of identity that is relevant here is the notion of “being one and the same as.” Suppose that a family member gave you a book, say a copy of Moby-Dick. You unwrap the book at your birthday party. You keep this book with you for very many years, not just because you like the story, but because that very book, the one that you unwrapped as a gift, has become precious to you. Suppose one day you lose the book. If you go to the store and purchase a replacement, is the book that you bought identical to the one you lost? It has the same story and perhaps the same number of pages, but it is not one and the same book. In the strict senses of “identity” and “identical” that matter for the present chapter, the book you purchased is not identical to the book you received as a gift. They are not one and the same book but, instead, two different books.

The relation that the second book bears to the first one is not identity but instead mere similarity. One way of distinguishing this strict sense of “identity” from looser senses of the word is in terms of a distinction between numerical identity and qualitative identity. In the case of the two books, the first book may be qualitatively identical to the second book (meaning that they share qualities—they are similar) but the first book is not numerically identical to the second book (they are not one and the same). When we talk of twins being “identical twins” what we really mean is that the twins are qualitatively identical. They are not numerically identical. If they were, they wouldn't be twins (because there'd only be one of them).

In discussing the mind–brain identity theory, it is also important to keep in mind a distinction between a priori identities and a posteriori identities. But before saying more about identity, let's give a rough indication of what a priori versus a posteriori amounts to. Putting this very roughly, we can say that the distinction concerns kinds of knowledge and that a priori knowledge is knowledge one can obtain prior to having a sensory experience whereas a posteriori knowledge is knowledge one can obtain only by having a sensory experience.

Let's consider an illustration. Consider the question of how many people are in the White House right now. Unless you are there and have just recently counted the number of people in the White House (or have been in communication with someone who has), you will not know the answer to this question. Now, consider this question: Is it true of the number of people in the White House that it is either equal to or greater than zero? If you think about it for just a moment, you'll realize that the answer must be “yes.” There can't be a negative number of people in the White House. In the case of the first question, the knowledge we are seeking is a posteriori. You can only know how many people are in the White House if you've counted them or been in communication with someone who has. Either way, you must have a sensory experience that ultimately connects you with the White House and the people in it. In the case of the second question, the knowledge sought is a priori knowledge, for we didn't need any particular sensory experience concerning the White House in order to figure out the right answer. As long as we know the meanings of the relevant words, words like “number” and “White House,” the rest can be figured out just by reasoning.

Now that we have a rough grasp of the distinction between the a priori and the a posteriori, let us turn to apply the distinction to identity statements, statements of the form “X is identical to Y” and “X is identical to X.”

Here is an a posteriori identity statement: “The murderer of Jones is the owner of the grocery store.” This is a posteriori because we would have to conduct some sort of investigation, an investigation that ultimately involves our senses, in order to figure out whether it was true.

Here is an a priori identity statement: “The murderer of Jones is the murderer of Jones.” No investigation is needed to know that this is true. It is just obvious that it is true. As long as we know the meanings of the relevant words, we can figure out the truth value of the sentence by reason alone, with no additional assistance from the senses. Here is another a priori identity statement: “The oldest son of Sandra Mandik is identical to the oldest male offspring of Sandra Mandik.” Again, no investigation is needed to figure this one out, just a knowledge of the meaning of the words in the sentence.

With these ideas in hand, we can now appreciate an important point about the mind–brain identity theory: All of the central identity statements relating mind and brain are a posteriori identity statements. Statements such as “pain is identical to c-fibers firing” is not supposed to be a priori. It is not supposed to be simply obvious or something you could just figure out as long as you knew the meanings of the relevant words. Instead, it is a statement that requires investigation to figure out whether it is true. More specifically, it is a statement that requires scientific investigation to figure out its truth. Mind–brain identity theorists see the relevant mind–brain identity statements as very similar to other sorts of identity statements found in the sciences. Such examples include “water is identical to H O,” “lightning is identical to atmospheric electrical discharge,” and “heat is identical to 2 average molecular kinetic energy.”

Arguments against Mind–Brain Identity Theory

We will examine three arguments against the identity theory. They are (1) the zombie argument, (2) the multiple realizability argument, and (3) Max Black's “distinct property” argument.

The zombie argument

Just about any argument that can be used as an argument for property dualism (chapter 3) can also be used as an argument against the mind–brain identity theory. One especially noteworthy example is what is known as the “modal argument,” especially the version formulated in terms of zombies. Since we went over this in chapter 3, we will not go into as much detail in the present chapter. But briefly, here is a sketch of the argument adapted to be against identity theory:

• Premise 1: If identity theory is true, then, since having a quale would be having a certain pattern of neural activation, it would be impossible to have that neural activation without having any qualia. (Another way of putting the same point is to say that if it is possible to have any given neural state without any qualia, then identity theory is false.)
• Premise 2: If something is conceivable, it is possible.
• Premise 3: Zombies are conceivable. Further, the sort of zombie that is conceivable and relevant to identity theory is a zombie that is just like you with regard to its brain and neural properties.
• Premise 4: Zombies are possible, that is, something could have all the same neural properties as you but lack qualia.

The multiple realizability argument

The gist of the multiple realizability argument can be stated simply, but the simple statement will require some supporting explanation. First, let's look at the simple statement.

If identity theory is true, then it must be true at the level of types, that is, each type of mental state is one and the same as a type of physical state. However, it is false that each mental state-type is identical to a single physical state-type, since mental state-types are multiply realizable by physical state-types.

That simple statement is unlikely to make much sense by itself. To help explain it, there are two ideas we need to grasp. The first is the idea of a type. The second is the idea of a realization.

Let's think about the general idea of types by considering the specific idea of types of words. Consider this question: How many words are in the sentence, “The dog bit the cat”? There are two ways of understanding the question. One way is to understand it as asking a question about how many types of words appear. The other is to understand it as asking how many word tokens appear. Understanding the question as being about types leads to this answer: There are four words in the sentence— “the,” “dog,” “bit,” and “cat.” Understanding the question as being about tokens leads to this answer: There are five words in the sentence—two tokens of “the” and one token each of “dog,” “bit,” and “cat.”

Let us now turn to apply the type–token distinction to mental states. Suppose that two different people, Jones and Smith, have each stubbed their toe and are both in pain. They are each in a state of pain. Each is in the same type of mental state, namely a state of being in pain. So, Smith's pain and Jones's pain are two mental state tokens of the same mental state-type.

The mind–brain identity theory is a thesis about types. The mental state-type pain is supposed to be identical to a physiological state-type. As philosophers of mind like to say, pain is identical to c-fibers firing. That is, pains in general are identical to c-fibers firing. The identity theory as we are here presenting it is not saying simply that Jones's pain is identical to Jones's c-fibers firing. That would leave open the possibility that Smith's pain is identical to something else—q-fibers firing maybe.

Let us turn now to consider the idea of a realization, a technical concept in philosophy.

To illustrate, let's begin by thinking about the relationship between water and the specific chemical elements of which it's made. I'm drinking a glass of water right now, and likely other people all over the world are also drinking water right at the moment that I write this. And here's something that we know about each of those samples of water—each of them is composed of two parts hydrogen and one part oxygen. This is why the chemical formula for water is “H O.”

There's only one way of arranging subatomic particles to give rise to water. The arrangement has to involve a 2-to-1 ratio of hydrogen to oxygen. Contrast this feature of water with a feature of the various containers that people drink water from. Cups, mugs, glasses, bottles, etc., are all suitable for drinking water. However, there is no single way of arranging microphysical particles to give rise to a drinking vessel. Some such containers may be made of metal and others may be made of glass. There's no single chemical that is required to make something that can serve as a drinking vessel. Multiple chemically distinct arrangements of particles can give rise to a container suitable for drinking.

We can summarize these different facts about water and containers in the following way. Containers are multiply realizable. Water is not. Containers have multiple physical realizations. Water has only one. There are multiple physical ways in which to realize a container. There is only one physical way in which to realize a sample of water.

We can relate the idea of multiple realizability to the ideas of types and tokens in the following way: The type sample of water is identical to the type sample of H O. Every token of the type will also be a token of the type . 2 sample of water sample of H O2 However, things are different when we turn our attention to multiply realizable types. The type vessel for drinking is not identical to any chemically specifiable type. For instance, the chemically specifiable type container made from aluminum is not one and the same as the type vessel for drinking since not every token of the type vessel for drinking will also be a token of the type container made from aluminum.

We are now in a position to understand the multiple realizability argument. Consider this question: Are minds and mental states more like water or more like drinking vessels? According to the multiple realizability argument, they are more like drinking vessels. They are multiply realizable. Consider an octopus and a person who are both in pain. These two different creatures have incredibly different nervous systems, so it's unlikely that the physical properties that give rise to pain in the octopus are of the same type as the physical properties that give rise to pain in the human. Despite these physical type differences, the human and the octopus are in the same type of mental state. They are both in pain.

The issues and ideas involved in the multiple realizability argument have been hugely influential in the philosophy of mind. They intersect with questions such as “Can machines such as computers think?” Computers are made of very different stuff than are humans. So, the question of whether mentality is multiply realizable is closely connected to the question of artificial intelligence. We will explore these issues and ideas further in chapters 7 and 8.

Max Black's “distinct property” argument

One famous argument against the mind–brain identity theory originates with the philosopher Max Black. Here's the gist: According to identity theory, the relevant statements of mind–brain identity are a posteriori. However, in order for an identity statement to be a posteriori, the two different referring expressions in the statement must be associated with distinct properties of the referent. This fact about a posteriori identity statements leads to property dualism, which is inconsistent with identity theory. That's the gist. Let's turn to explain this more carefully now.

Are you familiar with the star known as the morning star? How about the evening star? The morning star is a bright heavenly object that rises in the morning. The evening star is a bright heavenly object that rises in the evening. It turns out that neither is actually a star and that each is actually one and the same as the planet Venus, the second planet from the sun in our solar system. Now, consider the a posteriori identity statement “The morning star is identical to the evening star.”

How it is possible that this identity statement is a posteriori? What keeps it from being a priori like the identity statement “The morning is identical to the morning star”? Arguably, the key factor allowing an identity statement to be a posteriori is something about how the two different referring terms or referring expressions are each related to their referent. In this example the two different referring expressions are the phrases “the morning star” and “the evening star.” These referring expressions both have the same referent. That is, there is just one thing to which they each refer, namely the second planet from the sun, the planet Venus. And here is a proposal about how the two different expressions are differently related to Venus: Venus has two different properties, each of which is associated with just one of the two different referring expressions. The first property is the property of being a bright heavenly object that rises in the morning, and this property is associated with the expression “the morning star.” The second property is the property of being a bright heavenly object that rises in the evening, and this property is associated with the expression “the evening star.”

If this line of thinking is correct, then there always have to be two distinct properties associated with every a posteriori identity statement. We indicated earlier that when we apply this generalization to the central relevant identity statements of mind– brain identity theory we are led to a kind of dualism, and, of course, dualism is totally incompatible with mind–brain identity theory.

To see how this line of thought is supposed to lead to dualism, consider the identity statement, “Pain is c-fiber firing.” Here the two distinct referring expressions are “pain” and “c-fibers firing.” What distinct properties could plausibly be associated with these distinct expressions? In the case of “c-fibers firing,” the relevant property is some electrochemical property detectible via scientific methods. And plausibly, the crucial property associated with the term “pain” is the subjective painfulness of pain—a pain quale. But here's the problem: If a pain quale is distinct from neuroscientifically specifiable electrochemical properties, then that's property dualism.