If someone in the future travels back in time and alters history we couldn't know about (the alteration) it if it happens(ed)...

Since the "first time" could possibly erase the potential for it to happen again (or at all)... the suggestion that time is not linear by nature, only perspective, and that time travel (backward) is impossible, must follow.

...alternate histories theory...

Given the constant proof of "perception is reality" that flies in the face of logic... I'm starting to believe it...

edit-How fitting it was that the e-mail I had waiting for me after posting this was tsuwm's wwftd...