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Did Doc Brown always have the letter that Marty left him in 1955, and was that the reason why he was so insistent that Marty meet him at the mall?

In other words, he knew he had to make sure Marty went back in time, because it was already part of the timeline.

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You might want to look at this question of the SF&F SE which deals with the BTTF timelines. – BCdotWEB Mar 4 at 13:46
up vote 18 down vote accepted

In the original timeline, when the movie starts and Doc tells Marty to meet him at the mall, he doesn't yet know if the experiment will work. Time travel in Back to the Future works by changing history, and at that point, no one had time traveled yet.

In the new timeline, shown at the end of the movie, it's unknown whether all of the 1985 events from the beginning of the movie still happen--Marty might have driven to school rather than hitching rides, for example--but we do know that Marty meets Doc at the mall, the time travel experiment still happens, and the Libyans still show up. We also know that Doc had the letter (and the vest) in this timeline, though, where he didn't before.

As a result, it's unknown whether Doc in this timeline did the experiment because he knew he "had" to, or just to see if it would work like in the original timeline, or some combination of factors, but he does seem unsurprised to see Marty when he reappears back at the mall.

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I think that, in the beginning of the film, Doc really doesn't have the letter. Marty's actions in the past changed our present, which is visible by the changes in the characters of Marty's family members and Biff, by the "pines" sign, etc.

"Twin pines/Lone pine" sign

(image source)

There is no reason for the letter to be an exception.

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This very question was asked and answered (by BTTF writer, Bob Gale) on the film's official FAQ

Doc Brown of 1955 learns a lot about the future from Marty. Shouldn't the Doc of 1985 remember all of those things that happened in 1955?

A: 3 possible answers, all credible. 1) The "Ripple Effect" of time travel (which caused all of the photographs to change) does not affect human memory. 2) The 1955 Doc suffered a memory loss sometime after his adventures with Marty (maybe it was from the drugs he took in the 60's as Reverend Jim!). 3) Doc actually did remember everything, but he still did all the same things he "remembered" because he didn't want to risk disrupting the space-time continuum.

There's a 4th possibility which depends on your view of time travel. There's a theory (we like to call it the "Self-Preservation Instinct of the Space-Time Continuum Theory") that says that the continuum is always trying to keep itself "on course," and when things happen to change it, it always tries to correct itself. It is much like a river, which tries to keep its overall course. Although earthquakes, fallen trees, floods, or other circumstances might disrupt it at points, the river would cut a new channel so that it would end up back at the same place.

Thus, the overall physics (or metaphysics) of the space-time continuum would insure that any of Doc's memories of events that might create paradoxes would become hazy — or be erased.

To put it another way, he may well have always had the letter (hence the bulletproof vest), but his memory of the events that took place in 1955 would have only become clear as he got closer to the point that Marty traveled back in time.

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Yay for canon quotes! – rand al'thor Mar 6 at 11:50

I think Doc already knew. 2 things back this up in the movie itself - 1, at the beginning of the movie, we see a bunch of clocks - one of the clocks, show a man that looks like Doc hanging off a clock hand on a clock tower, much like Doc does later in the movie. Coincidence? Maybe. I mean, Doc would of had to construct that himself, right? Which brings me to point 2, when Doc and Marty tested their lightning bolt idea. Doc had an exact scale model replica of Town Hall with the streets and buildings surrounding it, which indicated that Doc was a talented sculptor, which means he very well was capable of making that clock in the beginning of the movie. So, I think he knew all along and just kept it to himself to not mess up time. Also, keep in mind, although we see Doc shot and laying on the ground when Marty travels back in time, we don't necessarily know for sure he died. He lays that way, and doesn't reveal he's still alive until after Marty #1 travels back in time and Marty #2 shows up and starts to mourn him. He could have been alive the whole time.

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