Engineer, physicist and mathematican are asked to find the value of 2+2.
Engineer (after 3 minutes, with a slide rule): "The answer is precisely
3.9974."
Physicist (after 6 hours of experiments): "The value is approximately 4.002,
with an error of plus-or-minus 0.005."
Mathematician (after a week of calculation): "Well, I haven't found an answer
yet but I CAN prove that an answer exists."