Abstract

Here is an old philosophical puzzle. Take out a coin and look at it. It is a flat disk. Now tilt it so that you look at it on an angle. From an angle, there is some sense in which the tilted coin appears elliptical. But there is also a strong sense in which the tilted coin looks circular, like a flat disk. How can one object look both elliptical and circular at the same time? Thus, the puzzle of the tilted coin