A mathematician went insane and believed that he was the
differentiation operator. His friends had him placed in a mental
hospital until he got better. All day he would go around frightening
the other patients by staring at them and saying "I differentiate
you!"
One day he met a new patient; and true to form he stared at him and
said "I differentiate you!", but for once, his victim's expression
didn't change. Surprised, the mathematician marshalled his energies,
stared fiercely at the new patient and said loudly "I differentiate
you!", but still the other man had no reaction. Finally, in
frustration, the mathematician screamed out "I DIFFERENTIATE YOU!" --
at which point the new patient calmly looked up and said, "You can
differentiate me all you like: I'm e to the x."