 Proper time is not a quantity attached to space-time points or to pairs of space-time points; it is in this respect a notion utterly different from the quantity “time” or “time interval” of pre-relativistic theory ... The fundamental physical role of proper time comes from the principle (here stated roughly) that whenever a process takes place along a well-defined line of space-time (“world-line”), the time rates in the dynamical principles that govern that process are to be understood in terms of the proper time along that line (and not in terms of a “time coordinate”...) (Stein, 1968, fn 6)