In my last blog posting, I discussed the view of the relation between mind and body that’s known as substance dualism—the theory that a person’s mind and body are two different things. The body (including, of course, the brain) is a physical thing, a configuration of matter, a flesh and blood machine, but the mind is a spooky non-physical thing that is not subject to the laws and forces that govern the physical world.  

If you’re attracted to substance dualism, and think that you and everyone else really are a compound of physical body and non-physical mind, then the next step is to try and explain how the two are connected. The most obvious strategy—in fact, the knee-jerk response—is to suppose that mind and body interact with each other: your body talks to your mind and your mind talks to your body on the model of a sort of metaphysical Wi-Fi. Our sense organs are the parts of our physical body that gather information about the (physical) world around us—the sights, and sounds, and smells, and feels that get woven together as the fabric of experience.  

If you’re an interactionist (someone who thinks that mind and body talk to each other) you think that these sensory inputs only become experiences once they’re transduced into information that’s transmitted to the non-physical mind. On the other side of the coin, you think that our decisions are made by the mind and then get transmitted to the body, which then does the mind’s bidding. So, as you read these words on your computer screen, what’s supposed to be happening is that your eyes are taking in the physical information, which then gets processed by your brain (still, totally physical) and then set off to your non-physical mind, where (BINGO!) you have the experience of reading and comprehending the blog posting. And when you decide to scroll down the page to read the next paragraph what’s supposed to be going on is that your non-physical mind makes the decision to scroll down and then sends the info to your physical body, which causes that body’s fingers to move in just the right way to scroll down the page.

This way of thinking about the mind-body connection is the default position among people who’ve never studied philosophy. It’s the general picture proposed by the 17th century French philosopher-physiologist-mathematician-physicist René Descartes (of “I think, therefore I am” fame). There were substance dualists long before Descartes came on the scene, and plenty more afterwards, but Descartes is especially closely associated with this philosophical theory. Think of him as “Mr. Substance Dualism.”  

But as compelling as the Descartes-style approach seems to be to many people, it’s now roundly rejected by most philosophers. Here’s why.

A woman named Elisabeth Simmern van Pallandt (also known under the monikers Princess Elisabeth of the Palatinate and Princess Elisabeth of Bohemia) fired the philosophical torpedo that sunk Descartes’ ship. What bugged Elisabeth was Descartes’ claim that non-physical things and physical things impact on one another. Why did she think that this is a problem? It’s because these two sorts of things are not supposed to have anything at all in common. There’s no shared territory where they can meet. But on that assumption it’s impossible for mind and body to come into contact with each other and it therefore seems impossible for things going on in our immaterial minds to make our material bodies do things, and it’s equally impossible for happenings in our fleshy bodies to generate experiences in our un-fleshy minds.

This might sound really obscure, so let me try to unpack it in a different way. Think of an abstract, non-physical object—say, the number 37. Because the number 37 is an abstract object, there are certain questions that we can’t meaningfully ask about it. Suppose that I asked you “Where was the number 37 at 2:15 PM last Thursday?” That question doesn’t have an answer. It doesn’t even make sense. It would sound completely crazy to say, for example, that the number 37 was in San Francisco at 2:15 on Thursday. How come? Well, numbers aren’t the sorts of things that can be in a place at a certain time because they don’t exist in space and time at all.

Moving on a bit, suppose that I said to you “The number 37 fell on my left foot this morning and gave me a nasty bruise.” That would be equally loony-tune. The number 37 would have to exist in space and time in order to have dropped on my foot, and we’ve already established that it doesn’t exist in space and time. But even worse, the number 37 isn’t the sort of thing that can come into contact with any material object whatsoever (including my left foot!). Of course, a block of wood or a stone sculpture that’s shaped in the form of the numeral 37 could fall on one’s foot and bruise it—but blocks of wood and stone sculptures are concrete material objects, while the number 37—the number itself—isn’t.

Now let’s get back to Princess Elisabeth’s criticism. Descartes didn’t think of the non-physical mind as an abstract object like a number, but his concept of the mind has something important in common with the number 37. Cartesian minds, like numbers, don’t exist in time and space, and that means that—just like numbers—they can’t bump up against physical objects to affect them: a non-physical mind can’t fall on anyone’s foot and bruise it any more than the number 37 can. But if all this is true—as it surely is—then how on earth could a non-physical mind “bump up” against a physical brain? And how could electrochemical impulses coursing through the mass of nerve tissue between your ears cause anything to happen in a mind that lies entirely beyond the material world?  

When you really think it through, Descartes’ version of substance dualism looks more and more like a non-starter.

Nowadays, there’s a fancy name for the problem that Princess Elisabeth identified. It’s called the problem of psychophysical causation. Over time, there were all sorts of ways that philosophers tried to solve it. One strategy, known as “psycho-physical parallelism,” is to hold onto the dualistic idea that bodies are physical and minds aren’t, but then go on to claim that they don’t really interact but only seem to do so.

A parallelist would say that when you decide to scroll down the page, and your fingers make exactly the right movements, it’s not because your decision caused your fingers to move, and when you whack your thumb with a hammer and you have the experience of it hurting like hell, the physical event of the hammer mashing your thumb didn’t cause the mental event of feeling pain. So what’s supposedly going on? The physical and mental events are just perfectly coordinated, running in parallel but never touching each other. When you crushed your thumb, the agony that you felt came after the crushing but didn’t come because of it.

And what could account for this amazing coincidence? Why God, of course! According to one version of this extravagantly weird idea, God set things up at the beginning of time so that physical and mental tracks are perfectly synchronized, like two clocks that are set to tell the same time. According to another version, God is at work 24/7 to make sure that everyone’s mental states match up with what their bodies are up to.

Psychophysical parallelism does solve Princess Elisabeth’s problem, but at a humongous cost. Descartes’ brand of dualism may be hard to swallow, but the parallelist’s purported solution is even harder to swallow.

However, there is another, much more promising way to address the problem—one that the princess was attracted to and which the majority of present-day philosophers are inclined to accept. It involves trashing substance dualism completely, and arguing that mental states just are physical states. This position (or, more accurately, collection of positions) is called “physicalism,” and will be the topic of my next piece on the Philosophy Talk blog.