This article is from the The Terminator FAQ, by Karsten A. Loepelmann firstname.lastname@example.org with numerous contributions by others.
According to classical physics, we have a classic paradox. No one wrote it,
everybody just memorized it. Assuming a multiverse, on the other hand, we
can posit that a future John Connor (whose mother encountered a Reese who
perhaps forgot the speech) *did* write the speech, and gave it to Reese.
Reese traveled back to 1984 in *another* universe, and gave it to Sarah
(which we saw in T1).