The “Hard Problem of Consciousness” is the problem of how physical processes in the brain give rise to the subjective experience of the mind and of the world. If you look at the brain from the outside, you see this extraordinary machine: an organ consisting of 84 billion neurons that fire in synchrony with each other. When I see, visual inputs come to my eyes—photons hit my eyes—they send a signal that goes up the optic nerve to the back of my brain, it sends neural firings propagating throughout my brain, and eventually I might produce an action.

From the outside, though, I look like a complicated mechanism, a robot. This is how science might describe me from the objective point of view. But there’s also a subjective point of view. There’s what it feels like for the agent who is seeing the scene. When I see you, I see colors, I see shapes, I have an experience from a first-person point of view. There’s something it’s like to be me. And this is the conscious experience of seeing. It’s part of the “inner movie” of the mind.

This inner movie has many, many dimensions. It has the dimension of vision, it has the dimension of sound—like a normal movie—but it also has touch, and taste, and smell. It has emotions, it has thought, it has a sense of one’s body. All of this is subjective experience. And it’s one of the most familiar facts in the world that we have this subjective experience, but it’s also one of the most mysterious. Why is it that these physical processes in the brain should produce subjective experience? Why doesn’t it go on in the dark without any consciousness at all? No one right now knows the answer to this question.

So, in the science and philosophy of consciousness, people use the word ‘consciousness’ for many different things. And some of the things are easier to explain than other things. So, sometimes I like to distinguish between the easy problems of consciousness and the hard problem. The easy problems aren’t really so easy, they’re as difficult as most problems in science. But it’s the hard problem that’s really the mystery. So, sometimes we use ‘consciousness’ just for the difference between being awake and being asleep. And we say, “Well, how is it that a system could be awake, and on its feet, and responding?” Maybe for this we can have an explanation in terms of the right neural mechanism in the head that produces this behavior. Eventually we’ll explain this; one of the easy problems. Or, how is it that the perceptual system can discriminate some information and respond to it? How is it the brain can monitor itself? Again, in principle that’s a question about objective processing: an explanation in terms of neurons in the brain could explain it. So, for all of these we have accepted paradigms from science come up with some mechanism in terms of neurons that produce this behavior, then we’ve solved the problem. So those are the easy problems.

The hard problem, by contrast, is the problem of how it is that all these processes give rise to subjective experience. And what’s distinctive about this is it doesn’t seem to be a problem about objective mechanisms, about—for example—behaviors that the brain produces. We could tell a complete story about the objective mechanisms in the brains, the neurons that fire, the behaviors they produce, and so on, explaining all these functions—awakeness, responsiveness, discrimination, monitoring—and there might still be a further question: why is all that associated with subjective experience? It’s just a different kind of question. Ultimately, it’s a question for science., but it’s a question which, right now, our scientific methods don’t have a very good handle on. So, at least for now, it’s a central question for philosophy.

Now, philosophy has a great history of turning its questions into science. Many of the great sciences started as areas of philosophy. Physics was originally part of philosophy until Newton developed his physical theories in the 17th century. Once he figured out how to solve it, then it became a science. Something similar happens with linguistics, and psychology, and economics. I would like, eventually, consciousness to turn into a problem for science. But first of all, philosophy needs to do its work with this problem to make it tractable.

Right now, we’re just in the stage where philosophy and science are cooperating on solving this problem. The scientists are now starting to think about consciousness for the first time in some years, but it’s still of a level of thinking about correlations—Correlations between the brain and consciousness—rather than the level of fundamental explanations. But we can hope that, eventually, we might have some theory of the connections between the brain and consciousness.

My own view, though, is that it will not be a purely reductionist or physical explanation. We won’t explain consciousness purely in terms of the brain. In terms of the brain you’ll get a good solution to the easy problems, to the various behaviors and so on, but you’ll never get a full solution to the hard problem: why is all this accompanied by experience? Rather, what we need to do is to take conscious experience itself as primitive, as a fundamental element of the world. It’s the same attitude we take towards space and time and mass in physics: we don’t reduce them to something more fundamental, but then we come up with fundamental laws that govern them and that can explain them.

So, in physics, the physicists say we’re looking for a fundamental theory in physics so simple maybe you could write it on the front of a t-shirt. For consciousness, maybe we want a story of the fundamental laws connecting the brain and consciousness so simple we can write it on the front of a t-shirt; the fundamental principles that connect brain processes to consciousness. Right now we don’t know what those fundamental principles might be. Eh, there are some ideas. The neuroscientist Giulio Tononi thinks it might be some principles involving integrated information, so that when the brain integrates, there’s a fundamental law that says when the brain integrates enough information, then you get consciousness.

This does raise many of the traditional aspects of the philosophical mind-body problem: what is the place of consciousness in the physical world and how does it exert a causal role? Is consciousness something separate from all the brain processes that exerts some effect on the brain, as many of the traditional dualists say?

A view I’m attracted to is that consciousness is actually present at a fundamental level inside the brain, and inside all physical processes. This is the traditional philosophical view known as panpsychism: that maybe all of physics involves some element of consciousness at the basic levels. Somehow, all of this composes to yield my consciousness. It’s a beautiful, unified, attractive view of the world where consciousness and the physical world might be integrated all the way down. The big problem for this view is, again, another version of the hard problem, but now in the version “how can these small bits of consciousness at the fundamental level add up to the kind of single, unified conscious experience that I have?” People call this the combination problem: how do the atoms of consciousness combine to yield human consciousness? Nobody, right now, has a solution to that problem, but at least people are thinking about it.

So right now, we’re in a real period—I think—of ferment and excitement with many ideas about how to solve this problem. Nothing like a consensus yet. I’d be surprised if we have a consensus solution to the problem in the next twenty or thirty years. But if we’re lucky, maybe within a hundred years it’s possible we could have some kind of accepted, philosophical/scientific theory of the hard problem of consciousness.