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Lightning has struck the rails on our railway embankment at two

places A and B far distant from each other. I make the additional

assertion that these two lightning flashes occurred

simultaneously. If I ask you whether there is sense in this

statement, you will answer my question with a decided "Yes." But

if I now approach you with the request to explain to me the sense

of the statement more precisely, you find after some

consideration that the answer to this question is not so easy as

it appears at first sight.