Physics professor has been doing an experiment, and has worked out an emphirical equation that seems to explain his data. He asks the math professor to look at it.

A week later, the math professor says the equation is invalid. By then, the physics professor has used his equation to predict the results of further experiments, and he is getting excellent results, so he asks the math professor to look again.

Another week goes by, and they meet once more. The math professor tells the physics professor the equation does work, "But only in the trivial case where the numbers are real and positive."