From William Flanagan's liner notes: "The slow movement, ANDANTINO QUASI ALLEGRETTO, is at once the most personal and the most original in structural procedure--and it is the very heart of the piece. An essentially independent introduction, which is a metamorphosed treatment of the third principal theme from the first movement, comes to a more or less complete rounding out in preparation for a delicate, undulating, sharply profiled theme introduced by the solo flute."
Flanagan was apparently so impressed by that flute motif that Flanagan himself used it in a piece he wrote called ANOTHER AUGUST. Which is another masterpiece. And I wish Bernstein had conducted it for the purpose of popularizing it.
Copland's greatest passage of music is the last 6 minutes of the 1st movement of SYMPHONY #3. After the loud brassy climax in the middle, it segues into a quiet section. It's the most achingly tender & wistful piece of music that I've ever heard. And only Copland could've written it.