Each of your eyes has a hole in it. Where the optic nerve leaves the back of the eye there are no light-sensitive cells at all, and the gap sits about fifteen degrees out from the centre of your gaze, toward your temple, wide enough to hold more than ten full moons side by side. Nobody knew it was there until the 1660s, when the French physicist Edme Mariotte worked out from dissections where a blind patch ought to be and then went looking for it, which is strange when you think how long people had been looking at things. It hides for two reasons: the other eye covers it, and the brain fills what is left with whatever surrounds it, so a blank wall stays blank and a patterned carpet stays patterned. Close your left eye and look at one word on this screen with your right, and a patch somewhere off to the right of it is being made up, convincingly, as you read.