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

If you want to be a physicist, you must do three thingsfirst, study mathematics, second, study more mathematics, and third, do the same.
Arnold Sommerfeld 
All integral laws of spectral lines and of atomic theory spring originally from the quantum theory. It is the mysterious organon on which Nature plays her music of the spectra, and according to the rhythm of which she regulates the structure of the atoms and nuclei.
Arnold Sommerfeld