Visual proofs, however, begin to fail when pictorial prototypes cease to represent the full meaning of the class of objects to which the proof refers. For instance, the difference between real numbers and rational numbers is difficult to represent visually (although I simulate it in some of my own software for schools, (Tall, 1991)). Here are two pictures.
The one on the left is of a continuous function on the rationals (the formula reads “if x2>2, then the value is 1 else it is –1, on the domain where x is rational). The one on the right is the real function taking the value x2(x2–1)+1 if x is rational, and 1 if x is irrational. It is continuous only at x=–1, 0 and 1. (It is even differentiable at x=0.)