Paul's explanation of where a few of his songs came from concerned dreams that he was having right before he awoke. One of the best known songs anywhere is Yesterday and the story (as retold on the metalfloss.com site) is as follows:
Paul McCartney awoke one night, went from his bed to a nearby piano, and played one of the most covered songs in music historyfor the very first time anywhere. He spent months asking people if they had heard the tune before, believing that he must have unconsciously plagiarized the tune from someone. Once he determined that he had indeed come up with it on his own, he started to work on the lyrics.
In other posts you will see more musicians talk about the dream influence in their work. You will note that many times the song is complete, in some cases it is just a melody - in all cases, the song was so important that the artist woke with a start and immediately wrote it down, played it, recorded it in some way.
We now know that they were probably in a REM state which allows very realistic dreams to take place. But a question remains. Sir Paul was concerned that the song melody was something he had picked up from some source that he could not remember. He spent months researching the tune to locate a source because he did not want to steal it in any way. He finally convinced himself that it was indeed his own inspiration so he finished it and recorded it.
The obvious question is - where did it come from? A few possibilities are his subconscious is deeply involved in his music and writes songs behind his back. Sometimes at night, in the dream state his subconscious plays songs for his conscious - and Sir Paul wakes and jots down what he heard because he consciously knows the tunes are really good. Another option is he is networking in his dreams with other people/sources that want to share inspiration with him while he sleeps and dreams. (There is some proof that this networking during REM is real)
We will post some other dream music from various artists to show that the phenomenon is possibly more common as we might imagine.