When I was young, I was fascinated by an optical illusion that you’ve probably encountered yourself: a diagram in which two identical parallel lines are made to appear different lengths by the addition of two short arrow-like fins at each of their ends, pointing inwards in the case of one line and outwards in the case of the other.
What was fascinating was the persistence of the illusion in the face of decisive knowledge that it wasn’t real. The error I made when I first saw the diagram – that I was looking at two lines of different lengths – was swiftly and easily corrected. But my sensory perception of what I was looking at wasn’t
