The rocker, who recently reteamed with Ringo Starr during a show in London, reveals the pair's late bandmates John Lennon and George Harrison are never far from his thoughts - especially when he's asleep.
Lennon was shot dead by assassin Mark David Chapman in 1980, while Harrison lost his battle with cancer in 2001.
"As a musician, you often have dreams about being in the studio or on stage, so I'm often with the guys," McCartney tells BBC Radio Scotland. "Just the other morning, I woke up and I was with George. And that was very nice. I think of George as my little mate - he was the youngest in the group."
"That's how I run into John and George these days. So the Beatles have reformed - in my head. Even though there is sadness, the main thing is the joy of knowing those two guys. I miss them a lot."
McCartney admits he'll always regret not reforming The Fab Four before Lennon's death, "It would have been great to get the Beatles back together again," he states. "People always say what if? But we can't, so, sadly, that's not going to happen."
Paul and Ringo reunited earlier this month (December 2018) onstage at London's O2 arena during the final night of the U.K. leg of McCartney's "Freshen Up" tour.