|Some People(jealous?} could not believe Berlin wrote all of those amazing songs on his own.
But he didn't. He wrote the basic tunes, but often needed help with harmonization and to create the accompaniment, and needed someone to transcribe the songs, because he didn't know how to read or notate music. He also had a piano with a shifting keyboard so that he could stick, when possible, to the black keys, and yet play a song in any key.