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 asphysicalism. Physicalism is the view thateverythingis 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 arekinds的事情。它们与符号——特殊的东西——形成对比。想知道区别，问问自己FOOD这个单词有多少个字母。如果你说有四个字母，你是在数token: F-O-O-D。但是如果你说有三个f O d，你是在计算类型。

现在让我们把这种区别应用到精神状态的例子中。假设你和我都认为今天是个晴天。你可以说我们的想法是一致的，因为我们都认为今天是个晴天。这就是计数类型。但你也可以说我们的想法不同(因为你的想法是你的，我的想法是我的)。计数的令牌。As I explained inmy March blog posting, when philosophers say that two things are identical, they mostly mean that these two things arereally 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—thatkind状态与处于某种神经状态相同——我们称之为“神经元a、B和C放电”(这是故意简化的)。如果这是正确的，那么每当有人想要啤酒时，这些神经元就会发光，而每当这些神经元发光时，被它们支配的那个人就会想要啤酒。心理类型(想要啤酒)与神经类型(神经元a、B和C兴奋)是相同的。即使希拉里没有说过她想喝啤酒，但如果我们把她放在脑镜(我自己的发明)里，看到神经元a、B和C被激活，我们就可以几乎100%地确定她想喝啤酒。

那么外星人呢?我们的外星朋友是一个物种的成员，他的进化史与我们地球人非常不同，因此他的神经解剖学与我们非常不同。他确实有神经元，但它们的结构和我们的非常不同。他的大脑——如果你想这么叫的话——分布在一个遍布全身的网络中，而不是局限在头骨内部。

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 withsomeneural 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.

令牌同一性理论比类型同一性理论更有效，但它仍然存在一个主要问题:它不能帮助我们理解终结者内部发生了什么。终结者没有任何神经元;他的脑子里装的是电脑而不是大脑。所以当他想喝啤酒的时候，这不可能是一种神经状态!

有一种物理主义的版本涵盖了《终结者》、《外星人》和《希拉里》。It’s calledfunctionalism. Basically, functionalism says that mental states are what brainsdo(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 whatonly大脑可以做。这就好比说，因为在路上开车是汽车的事，所以在路上行驶的只有汽车。“终结者”的计算单元就像主人想喝啤酒时的大脑一样，从功能主义者的角度来看，这就是为什么他想和两个朋友一起喝一杯冰啤酒的原因。

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.