The 77-year-old music legend puts a lot of time into re-learning their back catalog, because there is so much material he's penned over over the years that he's forgotten some of the notes.
McCartney is quoted by the Daily Mirror newspaper as saying: ''I have to re-learn everything.
''I've written an awful lot, you can't retain them all. ''We go in rehearsal and I'm, 'Oh yeah, that's how it goes.'''
The 'Let it Be' hitmaker even surprises himself at how ''clever'' he was with their songs.
Asked if he thinks their music is ''pretty good'', he replied: ''I do, I really do.
''Some of the old songs you say, 'Oh, that's clever, I wouldn't have done that.'
''It's exciting to think that still works.
''We were a little rock and roll group from Liverpool, it just kept going.''
Despite forgetting the odd notes and chords, McCartney isn't going to retire anytime soon.
The 'FourFiveSeconds' hitmaker is currently writing a children's book, which is being published in September, and is working on a stage version of the musical 'It's A Wonderful Life'.
And he will ''hopefully'' be working on another record.
He said: ''Writing music is still a thrill - out of nowhere you produce a rabbit. If you get one you like it's a great feeling.''
The music legend has no plans to follow in the footsteps of Sir Elton John by quitting touring the world, as getting on stage each night still brings him so much joy.
However, the 'Come On To Me' hitmaker previously admitted he did contemplate calling it a day in 2007, when he was 65.
That year, Macca completed just a mini six-date tour of intimate shows in the US and Europe to promote the release of the 'Memory Almost Full' album.
Asked if the thought has ever crossed his mind, he said: ''Inevitably you do, I mean, I had those thoughts at 65.
''Which is a while ago. 'Cos 65 is the retirement age. In my world, in the working class.''
When reminded that it has been pushed back to 66 now, he added: ''Oh they moved it? Hey, they can move it as far as they like, I don't mind.
'Cos my work is play. Seems to be working OK.''
The 'Hey Jude' hitmaker released his 17th solo album, 'Egypt Station', in September - his first album in five years, following 2013's 'New' - and he wrapped his 'Freshen Up' world tour earlier this month.
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