"'Let It Be' happened during a time when there was kind of a lot going on," McCartney said. "I think people were overdoing the use of substances. We certainly were.
"It was kind of common. It was the fashion. And anyone who remembers that time will know that. And I think I was getting, like, a little bit over the top with the whole thing - getting pretty tired and pretty wasted. And I went to bed one night and had a kind of restless night.
"But I had a dream where my mother, who had been dead at that point for about 10 years, came to me in the dream and it was as if she could see that I was troubled. And she sort of said to me, she said, 'Let it be.' And I remember quite clearly her saying ,'Let it be,' and 'It's going to be OK. Don't worry.' You know, 'Let it be.'
"I woke up and I remembered the dream, and I thought, 'Well, that's a great idea.' And I then sat down and wrote the song using the feeling from that dream and of my mum coming to me in the dream."
The song begins, "When I find myself in times of trouble, mother Mary comes to me, speaking words of wisdom, let it be."
Mary was McCartney's mother's name.
"When I put that in the song, I think a lot of people then thought that it's become kind of quasi-religious with Mother Mary, Virgin Mary. Which is fine by me, if that's how you want to take it.
"The actual reason was my mom came to me in a dream and actually said 'Let it be,' which turned out to be great advice. Thanks, mom!"
ANOTHER DREAM: "Let It Be" wasn't the only song that came to McCartney in a dream.
"'Yesterday' came to me in a dream, but at this time it wasn't just my mom saying a phrase," he said. "This was a whole tune that was in my head. I had no idea where it came from.
"Best I can think is that my computer [in my head] through the years loaded all these things and finally printed out this song in a dream kind of thing. I have to believe that that's magical. I have no other rational explanation for it."
NOT BAD: McCartney admits that, in recent years, he's performed more and more Beatles music on his concert tours.
"But I try and mix it with Wings, which is also very popular because there's kind of a younger generation out there that comes to our shows now," he said. "And then I mix that with stuff that is my kind of own solo stuff, but the largest proportion these days is Beatles stuff. Because, you know, I try and give audiences what they want, and a lot of that is Beatles stuff.
"It's not bad music."