For thousands of years, philosophers have tried to figure out the relation between mind and body. Until very recently the doctrine of substance dualism—the idea that minds are made out of spooky non-physical stuff, while bodies are made out of clunky matter—was virtually the only game in town. It was the theory of the mind-body relation that British philosopher Gilbert Ryle ridiculed as “the ghost in the machine.”

But by the middle of the 19th century, a new wind was blowing through the musty corridors of philosophy. Cutting-edge developments in physics, biology, and neuroscience were undermining the age-old dualist consensus.

In physics, the 1842 discovery of the law of conservation of energy (the principle that amount of energy in the physical universe remains constant) showed that non-physical minds can’t be in control of physical bodies because that would mean that fresh energy gets injected into the physical universe whenever anyone so much as lifts their little finger.

Next, in 1859, Charles Darwin lobbed the evolutionary bombshell at an unsuspecting world. Darwin made the case that we are animals, and that everything about us—including our fancy mental abilities—evolved in response to purely physical pressures. There’s no room in evolutionary theory for an immaterial mind, because non-physical things wouldn’t be subject to the forces of natural selection.

And hot on Darwin’s heels, the French anthropologist Paul Broca discovered an area of the brain that’s located just above the left ear where capacity for speech resides (and thirteen years later the German neurologist Karl Wernicke found a different region, towards the back of the brain, that’s responsible for speech comprehension). Ever since Descartes, philosophers had held up our ability to use language as evidence that we are more than merely physical beings. But these neuroscientific findings proved beyond a shadow of a doubt that the language capacity is, at the very least, intimately bound up with that fleshy bundle of nerve tissue between our ears.

By the late 19th century, things were looking bad for dualism. But most philosophers (and philosophically informed scientists) clung onto it with grim determination. Of course, there were some exceptions. It wasn’t till the middle of the 20th century that there was a sea change away from dualism and towards the doctrine known as physicalism. Physicalism is the view that everything is physical and therefore that supposedly non-physical things (things like spirits, ghosts, and disembodied minds) don’t exist.

There are three main versions of physicalism: type identity theory, token identity theory, and functionalism. Before plunging in to explain them, I’m going to have to offer you a two-minute refresher course on some basic philosophical concepts: the concepts of types, tokens, identity, and functions.

Types are kinds of things. They’re contrasted with tokens—particular things. To see the difference, ask yourself how many letters there are in the word FOOD. If you say that there are four letters, you’re counting tokens: F-O-O-D. But if you say there are three—F, O, and D—you’re counting types.  

Now let’s apply this distinction to the example of a mental state. Suppose that you and I are both thinking that today is a sunny day. You could say that that we’re both thinking the same thought, because we’re both thinking that today is a sunny day. That would be counting types. But you could also say that we’re thinking different thoughts (because your thought is yours and my thought is mine). That’s counting tokens. As I explained in my March blog posting, when philosophers say that two things are identical, they mostly mean that these two things are really the very same thing (Lois Lane thinks that Clark Kent and Superman are two different guys, but they’re actually identical). And when philosophers use the term “function” they usually use it to mean what a thing does, just as we do in everyday speech.  

Now lets pull all this stuff together, using a little story.

Hillary Clinton, E.T., and the Terminator walk into a bar, and plop themselves down on stools. The bartender asks them, “What’ll you have?”  “I want a beer!” says Hillary, E.T. pipes in with “I want a beer too!” and the Terminator intones (in a thick Austrian accent), “I also want a beer!”  Let’s think about what kind of physicalist theory we need to give a good account of what’s going on in the minds of these three very different kinds of beings.

Hillary’s desire for a beer is a pattern of neural activation in her brain. According to type identity theory, the state of wanting a beer—that kind of state—is identical to being in a certain kind of neural state—let’s call it “neurons A, B, and C firing” (this is deliberately oversimplified). If this is right, then whenever somebody wants a beer these neurons light up, and whenever these neurons light up, the person whose neurons they are wants a beer. The mental type (wanting a beer) is identical to the neural type (neurons A, B, and C firing). Even if Hillary hadn’t said anything about her craving for a beer, if we stuck her in a brainoscope (my own invention) and saw that neurons A, B, and C were activated, we would know with near 100% certainty that she wanted a beer.

Now what about E.T.? Our little alien friend is a member of a species with a very different evolutionary history than we earthlings have, and he therefore has a very different neuroanatomy than we do. He does have neurons, but they’re very differently configured than ours are. His brain—if you want to call it that—is distributed in a network that spreads through his whole body rather than being confined to the insides of his skull.  

If type identity theory is right, it’s got to be the case that the pattern of neural activation that goes on in Hillary’s brain when she wants a beer is the same (type-wise) as what happens in E.T.’s brain when he wants a beer. But that doesn’t even make sense! E.T.’s nervous system is so radically different from Hillary’s that no such comparison is possible. So if we want a physicalist theory that covers both the alien and the former Secretary of State, we have to drop type identity theory and move to something else.

Token identity theory will do the trick. Token identity theory states that every mental token is identical with some neural token. Whenever anyone wants a beer, that desire is identical with something going on inside their brain—but it doesn’t have to be the same thing from person to person or even from moment to moment.  Today, when Hillary wants a beer, neurons A, B, and C are firing in her brain, but last week when she wanted one that very thought was identical to neurons X, Y, and Z firing. And when she and her outer space buddy are sitting side-by-side, both wanting a beer, their nervous systems are in different states.

Token identity theory works better than type identity theory, but there’s still a major problem with it: it doesn’t help us understand what’s going on inside the Terminator. The Terminator doesn’t have any neurons; there’s a computer inside his head instead of a brain. So when he wants a beer, this can’t possibly be identical to a neural state!

There is one version of physicalism that covers the Terminator as well as E.T. and Hillary. It’s called functionalism. Basically, functionalism says that mental states are what brains do (this way of putting it is a little imprecise, as brains do lots of non-mental things too, but we can set that aside for now). When Hillary and E.T. wanted a beer, wanting a beer was what their brains, each in their different ways, were doing. But to say that mental states are what brains do isn’t the same as saying that that this is what only brains can do. That would be like saying that because driving down the road is what cars do, the only things that go down the road are cars. The Terminator’s computing unit does what brains do when their owners want a beer, and, from a functionalist perspective, that’s what accounts for the fact that he wanted to enjoy a cold one with his two friends.

You might think that this is all very far-fetched, because E.T. and the Terminator are merely fictional characters. But that would be too hasty. It’s very probable that there is intelligent life elsewhere in our vast universe, and very improbable that these alien beings have very different sorts of nervous systems than we do. And here on earth, advances in the field of artificial intelligence suggest that soon there will be thinking machines that can do what human brains do. If we ever make contact with interstellar aliens, or manage to build powerful artificial brains, philosophy—that great imagination-expander—will have already paved the way for understanding them.