If the me of the future is now in the past, how could you possibly know about it?
5 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:07:09.396 in the DVD version of the movie.
Quote context
[...]
- Doc, I am here, and it does make sense.
- Look, I came back to 1955 again with you, the you from 1985, 'cause we had to get a book from Biff!
- 1885?
- If the me of the future is now in the past, how could you possibly know about it?
- First, let me assure you that I'm alive and well.
- I've been living happily these past eight months in the year 1885.
[...]