As I was putting my 10-year-old son to bed tonight, as usual we had a long, free-ranging, unrushed conversation. Somehow I mentioned the missing Doctor Who tapes.

Peter got very thoughtful.

“If I had a time machine, I could go back to the moment those tapes disappeared. And I could bring them forward in time, so that they wouldn’t be lost. But there would be one problem. By going back in time to the moment the tapes disappeared, and keeping them from being lost, wouldn’t I be responsible for making them disappear? But I wouldn’t have ever gone back in time unless the tapes had disappeared.”

I told Peter he had stumbled across a closed causal loop — a concept that I introduce when I teach the play Oedipus Tyrannos. (In that play, the protagonist hears a prophecy that he will kill his father and marry his mother, leaves the court of his foster-father in order to escape the prophecy, kills a stranger who just happens to be his real father, and ends up marrying a widow who just happens to be his mother.)