Brains do many things: they detect light, store memories, steer attention and produce speech. Explaining how they do these things is difficult, but scientists have a clear idea of what success would look like. The philosopher David Chalmers, in a 1995 paper, called these the easy problems of consciousness, not because they are simple but because they are problems about mechanisms and functions.
The hard problem
The hard problem, in Chalmers' framing, is different. It asks why physical processes are accompanied by experience at all. Even after we explain how the brain discriminates colors and reports on them, a question seems to remain: why is there something it feels like to see red, rather than all that processing going on in the dark? Chalmers argued that no purely functional explanation could close this gap.
Nagel's bat
In a 1974 essay, Thomas Nagel asked what it is like to be a bat. Bats find their way by echolocation, a sense we lack. Nagel proposed that a creature is conscious when there is something it is like to be that creature. We could learn every fact about a bat's brain and still, he argued, not know what echolocation is like from the inside. Science describes the world from an objective standpoint, while experience is tied to a subjective point of view.
Qualia
Philosophers use the word qualia (singular quale) for the felt qualities of experience: the redness of a ripe tomato, the bitterness of coffee, the sharpness of a headache. The debate is over whether qualia are something extra beyond physical and functional facts.
Mary's room
Frank Jackson's 1982 thought experiment imagines Mary, a brilliant scientist raised in a black-and-white room. She learns every physical fact about color vision: wavelengths, retinas, neural pathways. Then she leaves the room and sees a red tomato for the first time. Does she learn something new? Many people feel she does: she learns what red looks like. If so, knowing all the physical facts may not capture what experience is like, and physicalism seems incomplete.
Zombies
A philosophical zombie is not a movie monster. It is a being physically just like you but with no experience at all. It talks about sunsets and winces at pinpricks, yet inside, so to speak, all is dark. Chalmers argued that such a being is conceivable, and that if it is even possible, consciousness cannot be just a matter of physical facts.
Physicalist replies
Physicalists have several responses.
- The ability hypothesis, defended by David Lewis and Laurence Nemirow, says Mary gains new abilities, not knowledge of new facts: the ability to recognize, imagine and remember red.
- Others say Mary learns an old fact in a new way. Someone who learns that water is H2O does not discover a new substance; she gains a new way of thinking about the same one.
- Against zombies, many physicalists argue that being conceivable does not prove something is possible. People once could conceive of water not being H2O, yet it could not have been otherwise.
- Philosophers such as Daniel Dennett argued that our intuitions about qualia are confused, and that once we explain everything the brain does, nothing will be left over to explain.
Interestingly, Jackson himself later became a physicalist. Meanwhile neuroscientists search for the neural correlates of consciousness, the brain activity that reliably accompanies particular experiences. Whether finding them would solve the hard problem, or only map it, remains disputed.
Worked example: describing coffee
Imagine trying to tell a friend who has never tasted coffee what it is like. You say it is bitter, a little like dark chocolate, roasted, warm. Your friend nods, but you both sense that the words do not hand over the experience. This is the gap the thought experiments in this level try to pin down.
Now suppose your friend is a brilliant food scientist, like Mary in her room. She knows every compound in coffee, every taste receptor it activates and every pathway in the brain that processes bitterness. Then she takes her first sip. Does she learn something new?
- A dualist or Chalmers-style reading says yes: she learns a fact about experience that no physical description contained. This supports the hard problem.
- The ability hypothesis says she gains abilities: she can now recognize coffee by taste, imagine it and compare it with tea. No new fact was missing from her science.
- The new way of knowing reply says she now grasps, from the inside, the very same brain state she already understood from the outside.
Then picture a zombie version of your friend, physically identical, who sips and says “mmm, bitter,” but experiences nothing. Can you really imagine that? Chalmers thinks so and draws a big conclusion. Physicalists reply that what we can imagine may not be genuinely possible, just as someone ignorant of chemistry might imagine water that is not H2O.
Notice what is at stake. If the dualist reading is right, science will need something more than physics to explain the taste of coffee. If the physicalist replies work, the mystery is about our concepts, not about the world.