Sentence:  Observing pi is easier than studying physical phenomena, because you can prove things in mathematics, whereas you can't prove anything in physics. And, unfortunately, the <WSD>laws</WSD> of physics change once every generation. 
Meaning of the word marked with <WSD> tags:  A statement (in physics, etc) of an (observed, established) order or sequence or relationship of phenomena which is invariable under certain conditions. (theory).