"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. It was published in 1941.

A man is visited by a stranger from the future via a time portal. The stranger pushes the protagonist back through the portal into the future. After a series of complicated events during which the protagonist meets up with himself several times, he gains control over the time machine that can create the portals, and uses it to set up that initial visit in the past. It is thus revealed that all of the characters who used the time machine were in fact the protagonist.