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.