In a current thread Why Expanding Space where a discussion of the Equivalence Principle (EP) of General Relativity (GR) would be off-topic, I said (perhaps unwisely) that the EP is something not understood. Even more unwisely, I said that it is statement raised to the status of a principle, so that one could avoid explaining why it is true. In reply atyy makes a nice distinction, namely that: . I take this to mean that the EP is not 'a truth used as a basis for a theory' (GR) but that it is a convenient and sufficient, but not necessary, assumption to be made for the purpose of developing GR. Or that if GR were to be developed without leaning on the EP as a kind of crutch, the EP would then emerge naturally as part of the structure of GR. I hope I haven't read into this distinction more than was meant -- apologies if I have, atyy. I'm wary of statements called Principles: however reasonable, they seem to me to have an ex cathedra flavour. For instance the EP is eminently reasonable (Einstein elevator logic) and is fully justified a posteiori by the predictive success of GR and by the excellence of that theory's internal logic. But couldn't there be more direct reasons for the truth of the EP, reducing its status to only a 'heuristic principle' rather than a 'principle principle', as atyy put it? Another example is the Copernican Principle: that the universe is everywhere much the same, which underlies modern cosmology. Why is this so? Well, it could be that everything --- perhaps physical laws included --- had a common origin, as many cosmologists postulate. In which case this also becomes just a well-supported-by-observation heuristic principle. I'd rather drop the label "principle" entirely rather that distinguish between different varieties of principle. Shouldn't one rather ask why these principles are the truths they appear to be?