Now we have a problem. We can deduce, often, from one part of physics like the law of gravitation, a principle which turns out to be much more valid than the derivation. This doesn't happen in mathematics, that the theorems come out in places where they're not supposed to be!

— Richard Feynman, The Character of Physical Law (1965)

Source: http://www.youtube.com/watch?v=M9ZYEb0Vf8U

Chapter 2, on the relation of mathematics to physics, discussing conservation of angular momentum.

Topics: mathematics