If you’re not familiar with what a paradox is then look it up or follow this example: You go back in time and accidentally kill your dad before your were born, but then you wouldn’t exist to go back in time to kill your dad, so he lives and gives birth to you, and you go back in time and accidentally kill your dad.
That’s a direct paradox, I could go into a diatribe about indirect paradox (e.g. you go back to the American Revolution and side with the redcoats, perhaps you still exist, perhaps not, but if you did would you still have gone back? Think of the show Sliders with how a change in history could impact how we live now.)
After seeing the movie Looper I discussed the paradox it created with my wife. Her response, “How do we know? Has anyone ever experienced one?” To which I could only respond, “touché.”
Take away: How do we know?