Arnold Sommerfeld quotes:

Mathematics is like childhood diseases. The younger you get it, the better.

Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you any more.

If you want to be a physicist, you must do three things-first, study mathematics, second, study more mathematics, and third, do the same.