I've been reading about about the isotropic and homogeneous nature of the universe, and as far as I can tell, it's basically a convention -- i.e., space is the same in all directions and the same in all locations only on average, over a very large area; observational data could one day reveal that the universe is in fact not isotropic or homogeneos. It occurs to me that our knowledge of the speed of light is similar: we don't (or can't) really know the one-way speed of light; 300,000 km/s is just a convention we adopt (based on our measurement of the round-trip speed of light). Does this have anything to do with symmetry and conservation laws? Does anyone have any thoughts on possible connections between the things I've just mentioned here? (I'm coming at physics from a philosophy background, so my technical knowledge is very limited, but it feels like there must be something interesting going on when STR and theories about the nature of space seem to depend in some important way on convention). Any ideas would be much appreciated! Thanks.