Another interesting velocity-related conundrum: It’s impossible to measure the one-way speed of light (from a source to a detector) because we have to synchronize the clocks somehow, which requires an algorithm based on the speed of light to account for time dilation. We can only measure the two-way speed of light (from a source to a reflector and back).

So, from the standpoint of pure math, isotropy of the speed of light is purely a convention. The argument can only be made based on, so to speak, philosophical reasoning.