Firstly, a little background: I'm a compsci major with a new found love of mathematics and particularly formal logic. To put it bluntly, I like symbols. Reality doesn't really give me a warm fuzzy feeling inside, but when I see that two algebraic equations are identical, I feel like god is smiling on me (and, as you would guess, when I make a mistake, it's like I'm being tortured by demons) Anyways.. How do physicists prove theoretical / abstract concepts? I understand that all principles must be held up to the light of physical evidence and measurement, but say that you don't have that. Say you're having a conversation with a friend, a thought experiment or what-have-you. You're just trying to convince them that what you say is true, but without the benefit of experiment. How do you do this without the use of formal logic like in mathematics? Is convincing another physicist more about experience with a long list of principles, gathered by old dudes over centuries, and just becoming familiar with them, or is there a way to derive secondary laws from the primary ones?