I googled this question first but there seemed to be a few different answers and I'm not sure which is correct. So take for example the theory of general relativity and the first law of thermodynamics, both are true, but why is one a theory and one a law? Why was Einsteins theory called theory of general relativity and not law of general relativity? Also what about Heinsenberg's uncertainty principle, is this still a theory or law or just a mathematical statement? Can you tell me when someone discovers something about the universe, what determines if it's classed as a theory or a law. Thanks.