Is there some way to define an inertial coordinate system without being cyclical (defining it with terms that require an inertial coordinate system to define)? For example if you refer to straight lines... straight according to what coordinate system? Or if you refer to velocity... that too is a coordinate system dependant quantity. Or if you refer to a force, which is also a coordinate system dependant quantity, how would you even define a force without referring to something depending on an inertial coordinate system. And so on... --------------------- EDIT: For clarity, I'm not worried about relativity being some kind of "trick of circular reasoning". I'm a physics student, not a crackpot. It's just that I and some other students noticed that people after start from "given an inertial frame...", and was wondering if one could define it outright. EDIT(2): Hmmm... I of course am looking for a definition doesn't make relativity a tautology though. EDIT(3): Oh, and the closest I've gotten so far is along the lines of: if we have a "standard clock", then we could use the second postulate (the constancy of the speed of light in an inertial frame) to define an inertial frame and the first postulate (to best match the original 'intention/wording' something like: the laws of physics written using the coordinates from an inertial coordinate system are the same regardless of the choice of inertial coordinate system) is still falsifiable. The problem is how do we define a "standard clock" without assuming we can use the second postulate? Maybe there is a better approach along different lines, but I haven't found one yet.