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In several episodes of Doctor Who (The Runaway Bride and the Wedding of River Song spring to mind) the Doctor says that he cannot cross his own timeline for fear of creating a paradox... it's obviously a mechanism so the writers can limit him and stop him always "going back and fixing things".

The spoilers below refer to "The Wedding of River Song" and the end of the Impossible Astronaut arc

However in The Wedding of River Song river is both on the beach and in the space suit

A Good Man Goes to War Spoiler

In fact Amy's actually pregnant with her too so she's there a third time!

Back to tWoRS spoilers

What's more the Doctor actually invited her to do it. Knowing that she was going to kill him he sent the blue envelope so she'd see her do it?

In this case isn't River crossing her own timeline? Why can she do it and the doctor can't?

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It's one of those tricky ones, it's a TV show but it's a sci-fi... I'm not a member of SciFi so I posted it here! – Liath May 9 '13 at 10:34
Not crossing one's own timeline is the first law of time (according to the Doctor as of The Three Doctors). The Doctor is perfectly capable of crossing his own timeline, but doing so may have undefined consequences. Usually things are fine as long as there are minimal/no paradoxes involved. In River's case, she never touched herself and never spoke to herself, which is tame compared to when the Doctor crosses his own timeline. – Pharap May 28 '14 at 11:20
up vote 4 down vote accepted

Rule #1: The Doctor lies.

Most of the time, when the Doctor says, "I can't...", he's eventually shown to really mean, "I really, really, really shouldn't, but if I really had to, or really wanted to, I could."

The most clear example of this is the 1964 story, The Aztecs. The Doctor spends most of that story adamant that time cannot be changed. This notion is quietly dropped as too limiting for the writers, with the in-universe result that the Doctor is basically caught by we the audience in a lie which he's never directly called on by his companions. Since then, he never makes this claim; indeed, he's more likely to admonish the opposite when confronted by a companion saying something like, "But the world didn't end in 1980!" (Sarah Jane Smith, Pyramid of Mars).

(Of course, keep in mind that, in the days the Classic Series was being made, television was seen as an ephemeral medium. The BBC rarely reran anything, there was no home video, and no notion there ever would be. Hence, continuity was really just not seriously considered!)

Similarly, we see in "The Waters of Mars" that even supposedly "fixed points" can be changed. It just doesn't necessarily turn out very well when they are.

And finally, there are three canonical occasions in the Classic Series in which the Doctor not only crossed his own time stream but actually shook hands with himself, aptly named The Three Doctors (1973), The Five Doctors (1983) and The Two Doctors (1985); and one semi-canonical occasion so far in the Modern Series, the short "Time Crash".

Lastly, there's some reason to infer that River Song's tangled timeline is, in fact, laden with paradoxes (thus not really giving the lie to the Doctor's statements).

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A fantastic answer - thanks! – Liath May 9 '13 at 10:36

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