Raise your hand.
It seems simple enough. You decide to move it, and your hand moves.
But what exactly just happened?
A thought or intention — something apparently mental — was followed by electrical and chemical activity in your brain, signals travelling through nerves and muscles contracting in your arm.
Now reverse the process. Touch something hot and physical signals travel towards your brain. You experience pain.
Somehow the mental world of thoughts, feelings and experiences is intimately connected with the physical world of brains and bodies.
But what exactly is that connection?
This is the mind–body problem, one of the oldest and most difficult questions in philosophy.
What Do We Mean by the Mind?
We know that we have bodies because we can see and touch them. Doctors can examine our organs, and neuroscientists can investigate the brain.
The mind seems different.
You have thoughts, memories, intentions, emotions and sensations. You can imagine a beach while sitting in a room. You can silently remember yesterday or worry about tomorrow.
Other people can observe your behaviour and examine your brain activity, but they cannot directly experience your thoughts.
There therefore seem to be two perspectives on a human being.
From the outside, you are a physical organism containing a brain.
From the inside, you are a conscious subject experiencing a world.
Are these simply two ways of describing the same thing?
Or are mind and matter fundamentally different?
That is the heart of the problem.
Descartes and the Two Worlds
One of the most influential answers came from the seventeenth-century philosopher René Descartes.
Descartes believed that mind and matter were fundamentally different kinds of thing.
The body belongs to the physical world. It occupies space and operates according to physical laws.
The mind, however, thinks, doubts, imagines and experiences. Descartes did not believe it could simply be reduced to physical matter.
This position became known as substance dualism.
It fits quite naturally with everyday experience. Your body feels like something you have, while your thoughts seem to belong to the person you are.
It also fits comfortably with many religious traditions in which a soul or mind can exist independently of the body.
But dualism creates a serious problem.
How Could an Immaterial Mind Move a Physical Body?
Return to the raised hand.
If your decision to move your hand is non-physical, how does it cause physical neurons to fire and muscles to contract?
And in the opposite direction, how can physical events in your body produce a mental experience such as pain?
If mind and matter are completely different substances, there needs to be some way for them to interact.
Descartes believed that this interaction occurred through the brain, and he particularly associated it with the pineal gland. But identifying a location does not solve the deeper problem.
How can something non-physical cause a physical event at all?
This difficulty has followed dualism ever since.
It helped encourage philosophers and scientists to consider another possibility: perhaps the mind is not a separate substance after all.
Is the Mind What the Brain Does?
Today there is overwhelming evidence that our mental lives are closely dependent upon our brains.
Brain injuries can alter personality. Drugs can change mood and perception. Anaesthetics can cause consciousness to disappear temporarily. Electrical or chemical changes in the brain can affect memory, behaviour and emotion.
Diseases of the brain can profoundly change someone’s ability to recognise people, form memories, control impulses or understand language.
This makes it increasingly difficult to treat the brain as merely a receiver used by a completely independent mind.
A common modern position is therefore physicalism: everything that exists, including the mind, ultimately arises from physical reality.
On this view, there is no separate mental substance.
The mind is what a functioning brain does.
That sounds straightforward — until we consider experience itself.
The View From Inside
Imagine a neuroscientist studying your brain while you look at a bright red flower.
In principle, the scientist might identify which areas of your brain become active, trace signals arriving from your eyes and observe neurons processing visual information.
But you have something the scientist cannot observe directly.
You experience red.
There is something it feels like to see that colour.
The same applies to the taste of chocolate, the smell of coffee, the pain of a headache or the feeling produced by a favourite piece of music.
These subjective experiences are sometimes called qualia.
This creates one of the deepest puzzles surrounding the mind.
Even if neuroscience eventually explains exactly how the brain processes colour, will that also explain why processing information is accompanied by the experience of seeing red?
That question leads directly into the separate philosophical problem of consciousness.
Could Mind and Brain Be the Same Thing?
One possibility is that mental states simply are brain states.
Just as water turned out to be H₂O, perhaps what we call thoughts, sensations and emotions will eventually be understood completely in terms of brain processes.
This family of approaches is known as identity theory.
There is something attractive about it. We already know that apparently mysterious phenomena can become less mysterious once their underlying mechanisms are understood.
Lightning was once interpreted in supernatural terms. Science eventually explained it as an electrical phenomenon.
Perhaps consciousness will follow the same path.
But critics argue that the comparison does not quite work.
We can observe water from outside and discover its molecular structure. Consciousness includes something else: the first-person experience of being conscious.
An explanation of what the brain does may not automatically explain why there is something it feels like to be that brain.
Is the Mind More Like Software?
Another influential idea compares the relationship between mind and brain with software and hardware.
The same computer program can, in principle, run on different machines. What matters is not the particular material from which the computer is made but the way information is processed.
This inspired a philosophical approach called functionalism.
According to functionalism, a mental state is defined largely by what it does — the information it receives, how it relates to other mental states and what behaviour it produces.
Pain, for example, might involve detecting damage, generating distress and motivating avoidance.
This possibility has important consequences.
If minds depend upon patterns of information processing rather than biological tissue specifically, then perhaps a sufficiently sophisticated artificial system could possess a mind.
But once again consciousness creates a complication.
A machine might behave exactly as though it were in pain.
Would it actually feel pain?
The difference between behaving intelligently and having an inner experience is one reason artificial intelligence has made the mind–body problem newly relevant.
Could Mind Be a Fundamental Part of Reality?
Physicalism and dualism are not the only possibilities.
Some philosophers have suggested that mind and matter might be two aspects of a deeper underlying reality.
Others have revived forms of panpsychism – the idea that extremely basic forms of mentality or experience may be fundamental features of nature rather than something that suddenly appears only when brains become sufficiently complicated.
Panpsychism does not mean that rocks think or that electrons have personalities. The idea is that the ingredients from which consciousness develops might already be present in reality at a very basic level.
It remains highly controversial.
Its attraction comes partly from avoiding another difficult question: if matter is entirely non-conscious, how does consciousness ever emerge from it?
But panpsychism creates problems of its own. Even if tiny components of nature possessed some primitive experiential quality, how could these combine to produce the unified consciousness of a human being?
There is no generally accepted solution.
What Can Neuroscience Tell Us?
The mind–body problem is no longer purely philosophical.
Modern neuroscience allows us to investigate the brain in ways earlier philosophers could barely have imagined.
Researchers can study which brain networks are associated with attention, memory, perception and decision-making. They can observe how damage changes mental abilities and investigate what happens to brain activity during sleep, anaesthesia and altered states of consciousness.
These discoveries increasingly reveal how dependent our mental lives are upon the physical brain.
But an important distinction remains.
Science can investigate relationships between brain activity and mental experience. Philosophy asks what those relationships ultimately mean.
If a particular pattern of brain activity always accompanies a particular conscious experience, have we explained the experience — or have we identified its physical counterpart?
Perhaps further neuroscience will eventually dissolve the distinction.
Perhaps a conceptual problem will remain.
We do not yet know.
A Brain Looking at Itself
There is something particularly strange about the mind–body problem.
The human brain is a physical object produced by billions of years of cosmic and biological evolution.
Yet that physical object has somehow become capable of wondering what physical objects are.
Matter has organised itself into brains that can study matter, construct theories of the universe and ask whether matter alone can explain the minds doing the studying.
In that sense, investigating the mind is unlike investigating almost anything else.
The thing doing the investigation is also the thing being investigated.
Our brains are attempting to understand brains from the inside.
That may partly explain why the problem is so difficult.
So Are We Minds or Bodies?
There is no single philosophical answer accepted by everyone.
Dualists argue that mind cannot be reduced to matter.
Physicalists believe mental life ultimately depends upon physical processes.
Functionalists concentrate on what mental processes do rather than the particular substance producing them.
Other philosophers explore idealism, panpsychism or theories in which mind and matter emerge from something more fundamental.
Meanwhile, neuroscience continues to reveal increasingly detailed connections between our brains and virtually every aspect of our mental lives.
Perhaps future science will show that the mind is entirely explicable in terms of the brain.
Perhaps consciousness will require us to rethink what we mean by the physical world.
Or perhaps the distinction between mind and matter will eventually prove to be the wrong way of framing the problem.
For now, one fact remains extraordinary.
A collection of physical cells inside your skull somehow allows you to see a sunset, remember your childhood, understand a sentence, fall in love, imagine something that has never existed — and wonder how any of this is possible.
Understanding how brain activity becomes a mind is therefore not simply a question about the brain.
It is part of the much larger question of what kind of reality we inhabit.
Where the Question Leads
The mind–body problem connects several of philosophy’s Great Questions.
If everything about the mind is determined by physical processes in the brain, we soon encounter the question: Do we have free will?
If our experience of the world is constructed by the brain, we must ask: What can we know about reality outside our own experience?
And once we ask why brain activity is accompanied by an inner world at all, we arrive at perhaps the greatest mystery of the subject:
This is also where philosophy increasingly meets science.
Neuroscience can investigate the brain. Psychology can investigate behaviour and cognition. Artificial intelligence may help us understand what kinds of information processing produce intelligent behaviour.
Philosophy asks how all these discoveries fit together — and what they tell us about what a human being actually is.
We know beyond reasonable doubt that mind and brain are intimately connected.
What we still do not fully understand is why that connection produces the experience of being us.