Written in 1967.
"It was perfectly true that when a man went into the past and returned to his own time, it was very much like a man visiting another town and returning to his own. What he did in either town or time was exactly as real as a ting done in his own city or his own time period. If somebody come out of the future on a visit, it would be part of the reality of the visited time that somebody from the furture had been there. It looked perfectly simple. But......"