Given my background of undergraduate physics, I have come to associate the word 'theory' with the description of a system which is approximate at best. For example, the theory of special relativity is called a 'theory' because no matter how many experiments we perform to test length contraction and time dilation, there's still the possibility that some experiment might turn out negative results. That's my understanding for the use of the word 'theory' to describe the concepts of special relativity and I hope I'm right. Now, I've tried to apply this understanding to the use of the word 'theory' in mathematics. The mathematical statements pertaining to sets form what is called 'set theory'; the statements relating to probability form 'probability theory'. However, these statements cannot be theories in the physicist's sense of the word, right? Because these discoveries can never be disproved or falsified. The results are eternally valid. So, why use the word 'theory' and not 'law' to describe these systems? I have been an undergraduate a long time w/o finding a satisfactory answer. I hope I can kick-start a discussion on this issue here.