Keywords: velocity, clock synchronization, PLSI
We can neither measure instantaneous speed nor synchronize separated clocks with absolute assurance. The reason is that velocity determination and clock synchronization depend on each other. Hence the realization that Special Relativity's Principle of light speed invariance (PLSI) is circular and self-fulfilling. PLSI can only base itself upon presumed (not affirmed) valocity/synchronization. In other words, Einstein's Second Postulate can only be a fiat, not a true find of nature.