For such a memorable cinematic friendship, it’s strange that nowhere in the course of the Back to the Future trilogy do we find out how Doc and Marty met in the first place. There are several obvious questions about how they became so close, despite clear differences in age, lifestyle, and common interests. But after the events of the films, there are a few more specific, paradoxical questions about exactly what Doc knows when he meets Marty “for the first time.”
In 1995, Doc sent Marty in a DeLorean time machine twice in a row, and in the process learned an absurd amount of details about his own future. He knows he’ll invent time travel someday, he knows how to prevent his own death in 1985, and he knows all about Marty’s future for the next few decades.
Since Doc isn’t the thinnest operator, how is he going to play all of this casually when he meets 80’s Marty? Will he just pretend not to remember anything throughout their friendship, or will he take a lot of drugs in the 60s and forget most of it? According to the logic of the films themselves, Doc and Marty are indeed “friends in time”, since their relationship itself is a chicken-and-egg paradox.