BY HIS BOOTSTRAPS

By His Bootstraps is a science fiction short story by Robert A. Heinlein that plays with some of the inherent paradoxes that would be caused by time travel.

A man is visited by a stranger from the future via a time machine. The stranger gives him the time machine and he uses it for a number of adventures, finally visiting himself in the past (and thus being revealed as having been the stranger). All of this was possible without any evidence that anyone had ever actually constructed the time machine in the first place.