Dear Charles

To: CHARLES FABIUS GRANVER,
Sector 233, Zone 3, Home 1254, Radli.
The Thirty-Fourth Century, a.d.
My dear great-great-great-etc.-grandson Charles:
Your friend Hari Vans will discover this letter printed as a fiction story in an ancient, tattered book of still more ancient fiction stories in the rare-books stacks of the University Library. He will be astonished to see your name and still more astonished to read his own. He will be astounded to find your correct address in a volume printed when neither you nor your address existed. So he will show this letter to you, and in this way I can write you a very important message. The ordinary postal service could hardly be expected to deliver a letter after fourteen centuries, and I feel I must tell you about urgent family matters.
I need to arrange, through you, to meet and woo (and of course to win, despite your unfilial objections) your great-great-etc.-grandmother. When this letter is delivered, she will happen to be engaged to you, so I do not really count on your co-operation. The most I expect is a frantic effort on your part to prove that the whole business is pure lunacy. But that effort will be all I need, Charles, and I think that for the family’s sake you should make it. It really is a family matter. As nearly as I can compute it on a basis of four generations to the century, you are my great-great-etc.-grandson some fifty-two times removed. This relationship exists because of a somewhat unusual series of events, and you need to know what to do to bring them about.
To make it clear… I imagine that in your day they still talk of time-travel as impossible because, so the argument runs, if one went back in time a hundred years, landed on his grandfather, and happened to kill him, he would make it impossible for himself to have been born. But of course if he wasn’t born his grandfather wouldn’t be killed. So he would be born. So he would kill his grandfather. So he wouldn’t be born. Ad infinitum. I am sure you know this proof that time-travel is impossible.