The grandfather paradox is a proposed paradox of time travel which results in an inconsistency through changing the past. The paradox is described as follows: a time traveler goes back in time and kills his grandfather, eliminating the possibility of one of the time traveler’s parents ever coming into existence.
Even without knowing whether time travel to the past is physically possible, the grandfather paradox can be explored from a logical perspective. The paradox is a logical contradiction that stems from changing the past (or the present, or the future) from the way it is: if an event has occurred one way, there is no possibility for the event to have occurred a different way. Then, logically, changing the past (or the present, or the future) from what it is results in a contradiction, which means changing the past is impossible. There still exists the logical possibility of travelling back in time and setting events the way they are, for example a time traveler intending to kill their own grandfather but instead killing someone who is not their grandfather.