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In order to have a complete description of the motion, we must

specify how the body alters its position with time; i.e. for

every point on the trajectory it must be stated at what time the

body is situated there. These data must be supplemented by such a

definition of time that, in virtue of this definition, these

time-values can be regarded essentially as magnitudes (results of

measurements) capable of observation. If we take our stand on the

ground of classical mechanics, we can satisfy this requirement

for our illustration in the following manner. We imagine two

clocks of identical construction; the man at the railway-carriage

window is holding one of them, and the man on the footpath the

other. Each of the observers determines the position on his own

reference-body occupied by the stone at each tick of the clock he

is holding in his hand. In this connection we have not taken

account of the inaccuracy involved by the finiteness of the

velocity of propagation of light.