A physics professor has been conducting experiments and has worked out a set of equations which seem to explain his data. Nevertheless, he is unsure if his equations are really correct and therefore asks a colleague from the math department to check them.
A week later, the math professor calls him: “I’m sorry, but your equations are complete nonsense.”
The physics professor is, of course, disappointed. Strangely, however, his incorrect equations turn out to be surprisingly accurate in predicting the results of further experiements. So, he asks the mathematician if he was sure about the equations being completely wrong.
“Well”, the mathematician replies, “they are not actually complete nonsense. But the only case in which they are true is the trivial one where the numbers are real and positive…”