This is the great mystery of human vision: Vivid pictures of the world appear before our mind’s eye, yet the brain’s visual system receives very little information from the world itself. Much of what we “see” we conjure in our heads. “A lot of the things you think you see you’re actually making up,” said Lai-Sang Young, a mathematician at New York University. “You don’t actually see them.” New research suggests mathematics is the key. The visual cortex has a mind of its own. “You may think of the brain as taking a photograph of what you see in your...