In 1994, philosopher David Chalmers stood before a conference in Tucson, Arizona and drew a line that has shaped consciousness studies ever since. He divided the problems of consciousness into two categories: the easy problems and the hard problem.
The easy problems — easy in principle, not in practice — are about explaining the functions of consciousness:
These are hard scientific problems, but they are problems of mechanism. In principle, neuroscience can explain them the way it explains any biological function — by describing the underlying processes. We might not have the answers yet, but we understand what kind of answer we're looking for.
Even if we solve every "easy" problem — if we map every circuit, explain every function, predict every behavior — we are left with a question that none of this addresses:
Why is all this processing accompanied by subjective experience?
Why does the wavelength of 700 nanometers feel red? Why does tissue damage hurt? Why is there an "inner movie" at all, rather than just information processing in the dark?
This is the hard problem. It's not about what consciousness does — it's about why it exists. A complete neuroscientific account of the brain could, in principle, explain every behavior a human produces without ever mentioning subjective experience. The hard problem is explaining why subjective experience is part of the picture at all.
Philosophers use the term qualia (singular: quale) to refer to the subjective, felt qualities of experience. The redness of red. The taste of coffee. The specific character of pain versus pleasure.
Qualia are what make the hard problem hard. You can describe the wavelength of light, the photoreceptors it activates, the neural pathways it follows, and the behavioral responses it produces. But none of this captures what it is like to see red. There is an explanatory gap between the objective description and the subjective experience.
Philosopher Frank Jackson (1982) posed this thought experiment:
Mary is a brilliant scientist who has lived her entire life in a black-and-white room. She has access to every possible physical fact about color — the wavelengths, the neural processes, the behavioral responses. She knows everything there is to know about how color vision works.
One day, she leaves the room and sees a red tomato for the first time.
Does she learn something new?
If yes — if knowing every physical fact about color wasn't enough to know what red looks like — then physical facts alone cannot fully account for consciousness. There is something beyond the physical description.
Chalmers asks us to imagine a philosophical zombie — a being physically identical to you in every respect. Same neurons, same chemistry, same behavior. It screams when cut, smiles when happy, says "I am conscious." But inside, there is nothing. No experience. No qualia. The lights are off.
Chalmers argues that zombies are at least conceivable. We can imagine a universe with all the same physics but no consciousness. If that's possible even in principle, then consciousness is not reducible to physical processes — it is something additional.
Critics counter that conceivability doesn't prove possibility. We can "conceive" of water not being H2O, but that doesn't make it possible.
Philosopher Joseph Levine coined the term "explanatory gap" in 1983. The gap is between:
No amount of third-person data seems to logically entail first-person experience. You can know everything about the neural correlates of pain without knowing what pain feels like. This gap may be a feature of our current ignorance, or it may be a fundamental limit of physical explanation.
Consciousness is entirely physical. The hard problem is real but solvable — once we understand the brain well enough, the explanatory gap will close. Qualia are brain states; we just don't have the conceptual tools to see how yet. Champions: Daniel Dennett, Patricia Churchland.
The brain is physical, but consciousness is a non-physical property that arises from physical systems. Chalmers calls this "naturalistic dualism": consciousness supervenes on the physical but is not reducible to it. It requires new fundamental laws, the way electromagnetism required laws beyond mechanics.
Consciousness is not something that emerges from complex systems — it is a fundamental feature of matter itself. Every physical system has some degree of experience. Electrons have a micro-experience. Brains have a macro-experience. The hard problem dissolves because consciousness was always there. Champion: Philip Goff.
The hard problem is based on a mistake. We think we have qualia, but introspection is unreliable. What we call subjective experience is itself a kind of representation — the brain modeling its own processes and generating the illusion that there's something it's like. Champion: Keith Frankish.
The hard problem isn't just philosophical navel-gazing. How we answer it determines:
After three decades, the hard problem remains unsolved. Some think it will eventually yield to neuroscience. Others think it marks a permanent boundary of human understanding. Either way, it is the deepest question we can ask about ourselves.