We measure the speed of light to be c because distance itself is defined by it (http://physics.nist.gov/cuu/Units/meter.html). If I declare that 1 foot = 10 bars and that 1 bar = 1/10 foots (and I have no other ways of measuring those quantities) we are simply employing circular definitions. Since c is distance over time, but distance is circularly defined by c, then it is the rate of passage of TIME which is the true invariant, and all of SR follows from that. Therefore, time is the TRUE invariant thing and not the speed of light. Do you see my logic here? Am I wrong?