Is there actually a superfluous reason for rest mass. I only ask because clearly as far as I know, all particles have spin, and im led, (possibly falsly) to assume that so do things such as photons (although we know photons dont have rest mass anyway). Thus if "everything" has spin, (lets just leave this as nucleons for now) then it has an acceleration, and therefore isnt at rest at all even without it accelerating by defintion. So is it therefore conceivable to suggest this notion of what we describe as rest mass is either nonsensicle or perhaps false convention if it existed as terminology before spin was introduced to our knowledge??