You are reading these words. Somewhere in your skull, electrochemical signals are passing among billions of neurons. At the same time you are having an experience: seeing shapes, understanding sentences, perhaps feeling curious or bored. How are these two descriptions related? That is the mind-body problem, one of the oldest and hardest puzzles in philosophy.

Descartes' dualism

René Descartes (1596-1650) tried to doubt everything he could. He found one thing he could not doubt: that he was thinking. In his Discourse on the Method (1637) he put it as “I think, therefore I am.” In the Meditations (1641) he went further. He could clearly conceive of his mind existing without his body, and he noted that bodies can be divided into parts while the mind cannot. He concluded that the mind is a thinking, unextended thing and the body an extended, unthinking thing.

This view is substance dualism: mind and body are two different kinds of thing. It has obvious appeal. It fits the religious belief that a soul can survive death, and it seems to explain why thoughts and feelings are so unlike lumps of matter.

The interaction problem

Dualism faces a famous difficulty. If the mind is not physical, how can it move the body, and how can a pinprick in the body cause pain in the mind? In letters of 1643, Princess Elisabeth of Bohemia pressed Descartes on exactly this: how could an unextended mind, which cannot touch anything, move the body? Descartes suggested that mind and body interact at the pineal gland, a small structure deep in the brain, but this only relocated the puzzle. Modern critics add that physics seems to find a sufficient physical cause for every physical event, leaving no gap for a nonphysical mind to push neurons.

Physicalism

The main alternative is physicalism (or materialism): everything that exists is physical or depends wholly on the physical. On this view mental states are brain states, or arise entirely from them.

An influential mid-twentieth-century version was behaviorism, which analyzed mental talk as talk about behavior and dispositions to behave. Gilbert Ryle's The Concept of Mind (1949) mocked Descartes' picture as the ghost in the machine, his phrase for the idea of a mind inhabiting and steering a body. Critics replied that pain seems to be more than a tendency to wince; an actor can wince without pain.

In the 1950s, philosophers such as U.T. Place and J.J.C. Smart proposed the identity theory: a sensation such as pain simply is a particular brain process, just as lightning turned out to be electrical discharge.

Functionalism

In the 1960s Hilary Putnam and others raised a problem for the identity theory: multiple realizability. Creatures with very different brains, such as octopuses, seem able to feel pain, and perhaps future aliens or robots could too. If pain just is one specific human brain state, they could not.

Functionalism answers that a mental state is defined by its causal role: what typically causes it, how it interacts with other mental states, and what behavior it produces. Pain is whatever state is caused by bodily damage and causes distress, avoidance and the wish for it to stop. A mousetrap can be made of wood, plastic or wire; what makes it a mousetrap is its job. A common, imperfect comparison says that mind is to brain as software is to hardware. Functionalism is widely held among cognitive scientists, but it faces a hard question, taken up in the next level: can a description of what a state does ever capture what it feels like?

Worked example: stubbing your toe

You walk across a dark room and stub your toe on a table leg. You feel a sharp pain, say “ouch,” and hop to a chair. Let us describe this one event in the terms of each view.

Substance dualism. The impact sends signals up your nerves to your brain. Then something remarkable must happen: a physical event in the brain causes a nonphysical event in the mind, the feeling of pain. That feeling then causes another physical event, the brain signals that move your mouth and legs. Princess Elisabeth's question arises twice: how does matter affect mind, and how does mind push matter?

Identity theory. There is only one kind of event. The pain simply is a particular pattern of activity in your nervous system, just as heat simply is molecular motion. The puzzle of interaction disappears, because there is nothing nonphysical to interact.

Functionalism. The pain is whatever state plays the pain role in you: caused by damage, producing distress, the “ouch” and the hop to the chair. In you that role is played by neurons. In an octopus, or in a well-designed robot, perhaps something quite different could play the same role.

Behaviorism. To be in pain is to be disposed to say “ouch,” limp and avoid table legs. But notice the problem: you might bite your lip and show nothing, and the pain would still be there.

Finally, ask what all the physicalist accounts leave out, if anything. Does describing the role or the brain process tell you how the pain feels? That question is the bridge to the next level.