Metaphysics · All lessons
You are, at once, a lump of physical matter and a stream of conscious experience — thoughts, feelings, the taste of coffee, the ache of regret. How can these be the same thing? The mind–body problem asks what the mind ultimately is and how it relates to the physical brain.
Two facts about yourself seem obviously true and strangely hard to fit together. First, you have a body and a brain made of ordinary physical stuff, obeying the laws of chemistry and physics. Second, you have an inner life: there is something it is like to be you, to see red, to feel pain, to want lunch. How does the grey matter in your skull give rise to — or relate to — that vivid first-person experience? And how can a mere thought ("I'll raise my arm") move a physical body?
Those two puzzles — how mind and matter connect, and how the physical could ever produce experience — are the heart of the mind–body problem.
Dualism holds that mind and body are two fundamentally different kinds of thing. The most famous version comes from René Descartes, who argued that the mind is a non-physical, thinking thing, wholly distinct from the extended, mechanical body. He noted that he could doubt he had a body, but could not doubt that he was thinking — suggesting mind and body are separable. (You can read the passage on the Discourse on the Method page.)
Dualism fits the strong intuition that experience is not just physics. Its great difficulty is the interaction problem: if the mind is non-physical, how does it push the physical body around, and how does light hitting the eye produce a non-physical sensation? Two utterly different kinds of thing seem unable to touch.
Physicalism (also called materialism) takes the opposite line: everything that exists is ultimately physical, and mental states are states of the brain and nervous system. Your feeling of joy is not accompanied by a pattern of neural activity — it is such a pattern, described from the inside.
Physicalism fits neatly with science, which keeps finding tight links between specific brain events and specific experiences. Its challenge is to explain how purely physical processes could ever add up to felt experience at all — a worry that has its own name (below).
Functionalism shifts the question from what the mind is made of to what it does. A mental state, on this view, is defined by its causal role — what typically causes it and what it typically brings about. Pain is whatever is caused by injury, makes you wince, and makes you avoid the cause.
A striking consequence is multiple realizability: if a mental state is just a role, then very different systems — a human brain, an octopus, perhaps one day a computer — could all be in the same mental state, as long as they play the same functional role. This idea is central to cognitive science and to debates about artificial intelligence. Critics counter that a system could play all the right roles and still, seemingly, have no inner experience.
The philosopher David Chalmers drew a line between the "easy" problems of mind — explaining how the brain processes information, controls behaviour, or reports its own states — and the hard problem: explaining why any of this is accompanied by subjective experience at all. Even a complete neuroscience, the worry goes, would tell us how the brain works while leaving untouched the question of why it is like something to be that brain. Whether the hard problem is a deep mystery or a confusion to be dissolved is itself hotly disputed.
Mary's room. Mary is a brilliant scientist who knows every physical fact about colour vision, but has lived her whole life in a black-and-white room. When she finally sees red for the first time, does she learn something new? If she does — what red actually looks like — then it seems there are facts about experience beyond all the physical facts. (Not everyone agrees; physicalists have many replies.)
Philosophical zombies. Imagine a being physically identical to you, behaving exactly like you, but with no inner experience at all — "all dark inside." If such a zombie is even conceivable, some argue, then consciousness must be something over and above the physical facts. Critics reply that zombies only seem conceivable because we do not yet understand consciousness.
These questions are no longer purely academic. If functionalism is right and minds are defined by what they do, then a machine that played all the right roles might genuinely think — and perhaps feel. If dualism or the hard problem is right, a machine might reproduce every behaviour of a mind while remaining an empty mechanism, with no one home. How we understand the mind–body relation shapes how we should think about the inner lives of animals and, increasingly, of artificial systems. Explore the competing views on the mind–body debate page.