What is the paradox of time?

A time paradox or an ontological paradox is something based on time travel that should not have occurred because the past had been changed, rendering it impossible for that event to occur.

For instance, John hates his grandfather and goes back in time to kill him, but his parents had not been conceived at the time of his death. Therefore, his parents should not exist, and he should not exist. He could not have gone back in time to kill his grandfather after all.

John might discover that the man he had killed was not his grandfather, or the entire universe might collapse as a result of the paradox.

There are also some other finite paradoxes, or closed loops that space does not allow to permit.