"The destiny trap: You can't change history if you're part of it."
— Doctor Who, "Time of the Doctor"
Time Agent #1: Captain, why did you take the Defiant back in time?
Sisko: It was an accident.
Time Agent #2: So, you're not contending it was a predestination paradox?
Time Agent #1: A time loop. That you were meant to go back into the past?
Sisko: Erm, no.
Time Agent #1: Good.
Time Agent #2: We hate those.
—Star Trek: Deep Space Nine, "Trials and Tribble-ations"
Our presence here doesn't alter history. You and I meet here because we are compelled to - we have always met here. History is irredeemable. Drop a stone into a rushing river - the current simply courses around it and flows on as if the obstruction were never there. You and I are pebbles, Raziel.
—Kain, Legacy of Kain: Soul Reaver 2
One of the major problems encountered in time travel is not... about changing the course of history — the course of history does not change because it all fits together like a jigsaw. All the important changes have happened before the things they were supposed to change and it all sorts itself out in the end. The major problem is quite simply one of grammar.