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After some time perhaps the following answer would occur to you:

"The significance of the statement is clear in itself and needs

no further explanation; of course it would require some

consideration if I were to be commissioned to determine by

observations whether in the actual case the two events took place

simultaneously or not." I cannot be satisfied with this answer

for the following reason. Supposing that as a result of ingenious

considerations an able meteorologist were to discover that the

lightning must always strike the places A and B simultaneously,

then we should be faced with the task of testing whether or not

this theoretical result is in accordance with the reality. We

encounter the same difficulty with all physical statements in

which the conception "simultaneous" plays a part. The concept

does not exist for the physicist until he has the possibility of

discovering whether or not it is fulfilled in an actual case.