Thought of this when I saw Terminator.
"Let us send a robot into the past to kill this dangerous man when he was a child."
"Yes sir...But captain...He is still alive right now. That means we must've failed to kill him with that robot."
"Holy crap!! You're right! Let us not waste our time then. We'll try something else..."
"But then if we don't send a robot, this person could be alive now simply because we didnt try to kill him that way."
"So have we already failed or not??"