5.3.2 Physicalism

Physicalism is the philosophy that states that the human person is made entirely of matter. As such it is a subset of materialism. Physicalism, therefore, claims that mental states/events are mere “brain events.”  Today we have the advantage of the far more extensive scientific study of the human brain. Experiments have shown that when a patient feels or experiences something specific, the neurons in a specific area of the brain “fire” to indicate that part of the brain is active. This leads many scientists to assume that our experiences are “nothing over and above” the relevant events in our brains. This straightforward identification of the mind with the brain is called identity theory, or identity physicalism.

Why do physicalist philosophers now generally agree that if physicalism is true, mental states and events will be identical to brain states and events? Why not claim, for example, that mental states and events are identical to physical states and events in our whole bodies, or even to physical states and events not only in our bodies but also in the world around us? For example, I see a book on the table. That is, I am having a conscious visual experience (a mental state) as of a book on a table. Doesn’t that mental state involve not just my brain, but my eyes, the book and the table, the light shining on them, and so on?

Roughly, the standard answer goes as follows: yes, of course, your mental state “involves” all of those things, but the mental state itself – the visual experience – is a phenomenon in the brain. What we seem to have learned – through a combination of ordinary experience, philosophical reflection, and scientific psychology (e.g., neuroscience) is that the book, the light, and even your eyes are not constituents of your visual experience, even if in this particular instance – they are the causes, or in the cases of the book and the table, are the contents (more below), of your visual experience.

We might place a blind person, say, in your exact physical situation: the same lighting conditions, surrounded by the same table and book, etc. Yet this person would not have any visual experience at all. Further, as Descartes argued forcefully in the Meditations, it is conceivable that you could have that exact same visual experience without the relevant book or table even existing. You could be dreaming, or hallucinating.

What we seem to see – again, as a result of ordinary experience, philosophical reflection, and neuroscience – is that mental states of all kinds are especially correlated with activities in the brain of the person whose mind it is. You can alter those other external facts in the ways discussed above without altering the visual experience. But changes in the relevant parts of the brain always seem to alter the relevant mental states, and all changes in the relevant mental states seem to go along with changes in the relevant parts of the brain.

And this is not just true for sensory experiences, but for thoughts, emotions, etc. Consider the famous cage of Phineas Gage. Gage was a railroad worker in the 19^th century who, in a terrible work accident, had a spike shoot through his brain. Miraculously, Gage survived, but according to those who knew him, he was a totally different person after the accident. Whereas before he was responsible and calm, afterwards he was a reckless gambler and a violent drunken lout.

A bad accident will have its effects, of course, whether it involves such extreme brain trauma or not. But this case is often used to show that changes to the brain can cause profound changes in the mental life of the person whose brain changes. There are many, many studies at this point that have shown relevantly similar results. Studies of people with brain lesions, seizures (and of people who have undergone certain surgeries to prevent or mitigate seizures – see the mention of ‘split-brain cases’ above), etc. have shown many different ways that the mind and brain are related.

In any case, these are some of the reasons why mind-brain identity theories have been more popular among philosophers than other theories that would try to reduce mental phenomena to physical phenomena other than (or in addition to) the brain.

Physicalist theories were especially popular in philosophy in the first half of the 20^th century, as the problems with dualism came to seem insurmountable, idealism had taken a rather obscure set of turns, and our evidence of the very close connections between mind and brain mounted. And identity theories clearly were not subject to any version of the interaction problem.

But a very serious problem was raised for such views in the 1960s and 1970s. This has come to be known as “the problem of multiple realizability.” Identity theory seems to imply that only creatures with brains can have minds. For example, if our pains are literally just certain neurons firing, then a creature that doesn’t have the same neural fibers would not experience pain.

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That seems wrong. It seems to be very easy to imagine creatures – aliens, angels, future robots, etc. – who might have all (or almost all) the same kinds of mental states that we do, but who do not have neural fiber brains. One famous example of the idea here comes from 20^th-century philosopher David Lewis (1941-2001). Here is his “Martian Pain” thought experiment:

There might be a Martian who sometimes feels pain, just as we do, but whose pain differs greatly from ours in its physical realization. His hydraulic mind contains nothing like our neurons. Rather, there are varying amounts of fluid in many inflatable cavities, and the inflation of any one of these cavities opens some valves and closes others. His mental plumbing pervades most of his body – in fact, all but the heat exchanger inside his head. When you pinch his skin, you cause no firing of C-fibers – he has none – but rather, you cause the inflation of many smallish cavities in his feet. When these cavities are inflated, he is in pain. And the effects of his pain are fitting: his thought and activity are disrupted, he groans and writhes, he is strongly motivated to stop you from pinching him and to see to it that you never do it again. In short, he feels pain but lacks the bodily states that either are pain or else accompany it in us (Lewis, “Mad Pain and Martian Pain,” Philosophical Papers Volume 1 (1983), p. 216)

If Lewis is right, then we must agree that mental states and events are multiply realizable: that is, they might be realized in different physical systems (or, as the dualist will point out, potentially in non-physical systems as well). Lots of things are multiply realizable. A dartboard, for example, might be made of cork, plastic, or wood. A couch might be made of cotton or leather or polyester. And so on. According to the objector, then, mental states/events are like dartboards or couches in the relevant respect.

Is the problem of multiple realizability a devastating problem for physicalism/materialism generally? Maybe, maybe not. It is certainly a serious problem for the most straightforward kind of physicalism, i.e., for what we earlier called identity theory. But over the last several decades, new, less straightforward physicalist-like theories of the mind have emerged.