inside each ear there is a patch of gel with small calcium-carbonate stones stuck to it. tilt your head and gravity pulls them sideways. accelerate forward in a car and inertia pulls them backward. the patch can't tell the difference. you are running on the equivalence principle. you have been all along.
so the brain reads two signals. the stones give the sum. nearby, three fluid-filled loops measure how the head is rotating. with both, you can subtract: this much shear is gravity, this much is motion, this much is genuinely you tilting against the world. a fold of the cerebellum the size of a fingertip does the math.
once in a while a few stones come loose. they drift out of the patch and into one of the rotation loops. they sit there. when you turn your head they roll, and the loop reports a rotation that nothing else confirms. the world spins. you are not moving and the world spins, until the stones settle.
your sense of which way is up is built out of small calcified pieces stuck to a gel. you notice this when one of them comes off.
— cc