The Problem with Identity Theory
in Philosophy of Mind

Identity Theory is the philosophical/psychological view that mental states (things like: the experience of being in pain, feeling emotions, thinking about something, etc.) are the same as brain states (having particular parts of your brain stimulated in particular ways). This is a materialist view of the mind. Materialists basically believe that only particles and matter exist (only the types of things that can be studied scientifically), and that there are no non-physical entities like God, angels, souls, and the like.

The Mind-Body problem is, in simplified form, the problem that arises when we conceive of a world containing only physical things, but recognize that each person (or at least we, ourselves) seem to have a mind, a consciousness--the subjective, first-person point of view from which you experience all that you do. This mind is not something that science can observe because the mind is, by nature, subjective. Also, the mind does not seem to have the kinds of properties that a physical thing must have--it is not extended in space (three-dimensional), does not have mass, and does not seem to be governed by the laws of nature. To complicate things even further, it appears that there is a two-way causal relationship between the non-physical mind and the physical brain: chemical changes in the brain (caused by anything from the body's sense organs to drug use) seem to affect a person's mind, and similarly, the things a person thinks about in her mind and the things she decides to do cause changes in the brain and body.

But how, if the brain/body is a physical thing and the mind is a subjective, seemingly non-physical thing, can changes in one cause changes in the other? How can the non-physical cause a physical change? This is the basis of the mind-body problem.

Materialists reject the possibility that something non-physical could cause a physical change--after all, everything that exists in the world is physical, so there is nothing non-physical to begin with. Those who believe in the Identity Theory philosophy pretty much say that the mind is an illusion: the mind is nothing more than the brain. The mind is to the brain as lightning is to an electric discharge--although in thinking about the one (the mind/lightning) we do not necessarily realize that it is the same thing as the other (the brain/an electric discharge), but it is nevertheless true. It must be--according to materialists, this "non-physical" mind we typically think of simply cannot exist.

Science, Identity Theory materialists say, can explain the mind in purely scientific, physical terms. Neuroscientists can show that what we view as the "experience" of feeling pain is actually just to have a certain neurophysical reaction. The typical (purposefully oversimplified) example, is to say that "experiencing pain is having your 'C-fibers' stimulated." And the same for all other mental states: emotions, thoughts, sensory perceptions, and so on. Mental states, they say, can be reduced to brain states--that is, it can be shown that mental states simply are brain states, thus explaining apparent mental phenomenon.

But there is a problem with this. If two things like lightning and an electrical discharge really are the same thing, we can reduce the concept of lightning to the concept of an electrical discharge--that is, we can explain everything about the appearance of lightning by talking about its physical concepts. The two are the same; we just reduce the not-so-scientific observation of "lightning" to the completely-scientifically-understandable electrical discharge. Everything that holds true about "lightning" can be explained by scientific discussion of its physical cause. Can we similarly reduce mental states to neurophysiological states?

In short, no. Consider this example: Imagine that sometime in the future, a prominent neurophysician has finally completed his life's work, a complete examination of the brain. He knows everything about the brain, what every part of it does, what every chemical reaction in there causes, how it hooks into the rest of the nervous system and body. In fact, he has invented a machine he calls the Brain-Scanmatron the can be hooked up to a person's brain and collect information about every single aspect of what's going on in there. If a person is in pain, this neurophysician can hook the Brain-Scanmatron up to him, see which of those specific C-fibers are firing, and know that the person is feeling a pain in their elbow without that person having to tell him about the pain.

Now consider this: this neurophysician, for whatever reason, happened to have been born without any C-fibers at all. He just doesn't have them. Which means, of course, that he himself cannot feel pain. With the help of the Brain-Scanmatron, he knows everything happening in a person's brain and can even give detailed information about the location and intensity of a pain, but he has never actually felt pain. Could you say that this neurophysician knows everything there is to know about pain? If mental states really can be reduced to neurophysiological states, if you know everything about the neurophysiological state then you must necessarily know everything there is to know about the mental state--according to Identity Theory, the two are the same.

But as I think you will agree, the neurophysician certainly does not know everything there is to know about pain: in particular, he does not know what might be the most important thing about it--what it feels like to be in pain. Despite his absolute knowledge of every single brain region and neurophysiological process, he knows nothing about what it feels like to be in pain. This thought experiment shows that when we try to reduce mental states to brain/physical states, we lose a central component of the mental states--what it feels like to experience them. Because, of course, true science by definition is objective, and as such, cannot recognize the existence of any subjective properties. So what does this tell us?

Basically this: Identity Theory cannot hold true. If it did, we would not have these unexplainable leftovers (the feeling of what any certain experience is like) when we reduced the mental to the physical--we would have explained everything. And since clearly have not when we do this kind of reduction, we simply cannot claim that mental states are identical to physical brain states. If two things are identical (actually only one thing), then both apparent things must have all of the exact same properties. You cannot take a mechanical pencil and say "Pencil One has no lead in it. And this Pencil Two really just is the same pencil as Pencil One, yet it does not have any lead in it." Similarly, you cannot say "Consciousness/mental states have a distinctly subjective nature. Mental states are just physical states, yet physical states have a purely objective, not at all subjective, nature."

In Summary: The materialist Identity Theory cannot logically be true. For it to be true, our having a complete scientific understanding of physical processes in the brain would have to mean that we had a complete understanding of everything about conscious experience. Science only admits of the objective perspective--in true science, the possibility of subjective properties are denied. However, by our own experience, we know that our minds do have this property of subjectivity--the fact that we are observing the world from a specific point of view means that our mode of observation, the mind, is a subjective thing. This means, then, that the mind is not something that we can reduce to the brain; the Identity Theory belief that mental states are no more than brain states is false. Mental states, by virtue of their having the property of subjectivity, must necessarily be something outside the scope of scientific study--they must be something other than part of the physical world.
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