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[7] We suppose further that, when three events A, B and C occur

in different places in such a manner that, if A is simultaneous

with B, and B is simultaneous with C (simultaneous in the sense

of the above definition), then the criterion for the simultaneity

of the pair of events A, C is also satisfied. This assumption is

a physical hypothesis about the law of propagation of light; it

must certainly be fulfilled if we are to maintain the law of the

constancy of the velocity of light in vacuo.