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 view is really not sound, because the universal time we are endeavouring to define is essentially quantitative in character. The best way of establishing the existence of a consistent method of comparison is to give an example of one, and so we shall consider Galileo's method of light signals, which was used in a first but unsuccessful attempt to measure the velocity of light.

The way in which this method is applied is as follows. An observer situated at a point A observes at time t an event which has taken place at another point B. If &tau; is the time which light takes to travel from B to A, the universal time to be associated with the event at B according to A&apos;s measurements is $$t-\tau$$. As soon as A has observed the event he makes a signal, and it is clear that by a series of signals the two measures of an interval of time may be compared.

By means of this rule the clocks belonging to a number of observers $$A_{1}, A_{2},\dots$$ can be regulated in a consistent manner, provided light always takes the same time to travel from an observer $$A_r$$ to an observer $$A_s$$.

Let us suppose that a large number of observers $$A_r$$ find that their observations of one another's experiences give a consistent universal time as far as they are concerned; they can then regard themselves as being at constant distances from one another, the distance between two observers $$A_{r}, A_{s}$$, being defined as $$C\tau_{rs}$$, where $$\tau_{rs}$$ is the time light takes to travel from $$A_r$$ to $$A_s$$, and C is a constant called the velocity of light.

These observers may then form a standard system for the measurement of time and distances at other points of space. The measurements of four standard observers should suffice to determine the position and time of any