What is consciousness?

The big idea: Bite into a lemon. There's a sharp, sour taste — and there's something it is like for you to have it.

That feel is consciousness: That 'what it's like' feel, right now, is consciousness. It sounds like the most obvious thing in the world, until someone asks you to explain it — and you can't.

Philosophers give the feel a name: qualia. The sourness of the lemon, the ache of a bruise, the exact blue of the sky you see — each has a feel that is yours.

Hold onto this: Consciousness isn't being clever or awake in general — it's that there's something it is like to be you from the inside. A rock has no inside feel. You do.

The sharpest way to feel the puzzle comes from the philosopher Thomas Nagel.

Nagel: the bat: A bat finds its way in the dark using echolocation — it squeaks and 'sees' the world by the echoes bouncing back. Nagel asks: what is it like to be that bat, sensing the world by sound?

The inside view: You could learn every fact about a bat's brain and its squeaks. But you still wouldn't know what it feels like from the inside to be one. There's an inside view that all the outside facts miss.

Checkpoint — Nagel: In one line: there's something it is like to be conscious, and knowing all the brain facts doesn't tell you what that is. The next thinker turns this into a sharper trap.

The philosopher Frank Jackson built a story that makes the gap impossible to ignore.

Mary's room: Mary is a brilliant scientist who has lived her whole life in a black-and-white room. She has learned every physical fact about colour — the wavelengths, the eye, exactly what the brain does when someone sees red.

The red rose

One day she walks outside and sees a red rose for the first time. Does she learn something new?

The gap: Most people say yes — she finally learns what red actually looks like. But she already knew all the physical facts. So the feel of red must be a fact the physics left out.

Go further — higher-level insight: Nagel's bat and Jackson's Mary are two versions of one move — the knowledge argument: complete physical knowledge still leaves out the feel. The sharpest reply is that Mary gains a new ability (knowing-how), not a new fact (knowing-that). Naming that reply is a top-band move.