Science has explained fire, lightning, heredity, and the origin of species. It has not explained why any of it is accompanied by an inner feeling — why there is something it is like to be you, right now, reading this.

The easy problems (which aren't easy)

In 1994 the philosopher David Chalmers drew a line that reorganised the whole subject. On one side he put what he provocatively called the easy problems of consciousness: how the brain tells colours apart, integrates information, focuses attention, reports its own states, steers behaviour. These are "easy" only by comparison — they are the work of centuries — but they are the right kind of problem. Each asks how the brain performs some function, and in principle a good enough account of the machinery answers it.

On the other side is the hard problem: why is all that functioning accompanied by experience at all? Why isn't the brain simply a superb computer processing colour in the dark, with no one home to see the red? A mechanism that did everything your brain does but felt like nothing — a "philosophical zombie" — seems at least conceivable. That you are not one is the deepest fact you know, and the one science has no grip on.

What it is like to be a bat

The sharpest statement of the puzzle came earlier, in 1974, from Thomas Nagel, in an essay with an unforgettable title: What Is It Like to Be a Bat? A bat navigates by sonar, sensing the world in a way you never will. You could learn everything physical about its brain — every neuron, every pulse — and still not know what it is like, from the inside, to perceive by echo. That inner, first-person character of experience — philosophers call the individual flavours qualia, the redness of red, the sting of salt — is exactly what a list of physical facts seems to leave out. Joseph Levine named the space between the two the explanatory gap: even a complete physics of the brain doesn't seem to entail why it feels like anything.

First, clear away a decoy. The gap is not a little screen in the head where the "real you" sits watching the show — the philosopher Daniel Dennett calls that the Cartesian theatre, and it only shoves the question back a step: who watches the watcher? There is no inner cinema and no little person inside. That makes the problem harder, not easier.

The candidate answers

Nobody has closed the gap, but the attempts sort into a few camps:

• It's all physical, and the mystery is a trick of perspective. Most neuroscientists work here. The illusionists, like Dennett, push further: the sense that there's an extra, ineffable something is itself just another brain process, and once you have explained the reporting, nothing is left over to explain.
• Experience is a further fact. The dualist holds that mind is genuinely not captured by physics. Few scientists accept this, yet no one has refuted the bare intuition under it.
• Experience goes all the way down. Panpsychism, an old idea taken newly seriously, proposes that some faint glimmer of the mental is a basic feature of matter, and brains merely concentrate it. Strange — but it sidesteps the trouble of getting mind out of utterly mindless parts.

Running beneath all of this is the empirical hunt Francis Crick and Christof Koch launched: pin down the neural correlates of consciousness, the exact brain activity that reliably accompanies an experience. That search is real progress. But a correlate tells you when the lights are on, not why there are lights.

The honest state

We can switch consciousness off with a drug and back on again, millions of times a year, without being able to say what we switched. We can watch which circuits carry it. We can trace, spike by spike, the physical machinery underneath. And some hope for an exotic escape hatch — that the answer hides in quantum machinery inside neurons — though the physics gives little reason to expect it.

We have mapped the machine in extraordinary detail and still cannot say why the mapping is accompanied by a point of view. That may be the largest open question in all of science.

It is not a gap in some distant galaxy or the universe's first microsecond. It is sitting behind your eyes as you read this sentence — and you are the only one who can confirm it is there.