Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory.
Stephen HawkingMaybe I'm being a bit harsh on philosophers, but they have not been very kind to me... I have been variously called nominalist, an instrumentalist, a positivist, a realist, and several other ists. The technique seems refutation by denigration: If you can attach a label to my approach, you don't have to say what is wrong with it... I am sure that Einstein, Heisenberg and Dirac didn't worry about whether they were realists or instrumentalists.
Stephen Hawking