Two medical professionals.
Also, two or more contradictory positions executed simultaneously.
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- "Oh, a lesson in not changing history from Mr. I'm-My-Own-Grandpa! Let's get the hell out of here already! Screw history!"
— Professor Hubert Farnsworth
File:Emot-science.gif Science warning: Scientific Stuff Ahead! Read at your own risk
The grandfather paradox is a proposed paradox of time travel first described (in this exact form) by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent (The Imprudent Traveller). Nevertheless, similar (and even more mind-boggling) paradoxes had already been described, for instance by Robert A. Heinlein in "By His Bootstraps". The paradox is this: suppose a man travelled back in time and killed his biological grandfather before the latter met the traveler's grandmother. As a result, one of the traveler's parents (and by extension the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which means the grandfather would still be alive, and the traveller would have been conceived allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.