Time travel gives rise to all sorts of strange situations called bootstrapping paradoxes. The famous example is Robert Heinlein’s story ‘–All You Zombies–’ wherein the protagonist, who undergoes a sex change and travels through time, is both their own mother and father (the just-released Australian film Predestinationis directly based on it). The person ‘comes from nowhere’; they ‘bootstrap’ themselves into existence. Or perhaps (spoiler alert!) you've seen the recent blockbuster Interstellar: there mankind is set on a course for extinction but humans from the future interact with the past to save mankind – they ‘bootstrap’ the survival of the entire human race. Similarly there are ‘information paradoxes’ where information appears only through the use of time travel e.g. a time traveler coming back from the future and telling themselves how to build a time machine.