So a time traveller says to his partner "time travel was a mistake, so I'm gonna go back in time to stop time travel from happening." His partner says "you already failed." "Why?" says the time traveller. His partner says "Well, if you're ever going to go back in time to stop time travel from happening, how are you talking about doing it now if you're going to succeed? If you stop time travel from happening, you wouldn't be able to go back in time to stop it from happening!"