In the National Gallery this morning, I was looking at a Madonna and Child by Antonio Rossellino (I show a detail here). It’s a low-relief sculpture carved about 1475. Look at the pillow at the bottom right, on which the toddler Jesus stands. This is a representation of a squarish object, depicted in linear perspective. To work correctly as a representation, it must be viewed from straight ahead. The marble pillow is also a three-dimensional object. You can look at it from several angles. If you were allowed to take it down off the wall, you could hold it, feel it, slide your finger behind it. It would not be a square but kind of a fat trapezoid. There is something quite strange about a three-dimensional object whose purpose is to look like an object of a different shape if viewed from a particular angle. What shape is it really?