By Jordan Runtagh
Updated August 10, 2016 at 09:37 PM EDT
Credit: Brian Killian/WireImage; Dominique Charriau/WireImage

Sir Paul McCartney surprised many fans when he teamed up with rapper Kanye West for a series of songs in early 2015. Now in his upcoming cover story for Rolling Stone, the rock icon revealed what it was like inside their musical collaboration.

West is known for being headstrong when it comes to his music, but McCartney insists that it was a true partnership. And before he agreed, the former Beatle had some terms.

"The only deal I made with Kanye was that if it doesn't work, we won't tell anyone," he says. "I didn't know his system. I'd heard things like, ‘He's got a room full of guys working on riffs, and he walks around going, "I like that one."‘ It reminded me of Andy Warhol, these artists who use students to paint their backgrounds and things. It's a well-used technique. I thought, ‘I don't know how I'm going to fit into that, but let's see. Here goes nothing.'

At one of their earliest sessions, McCartney shared a fragment of music he composed back in 1969 and had never fleshed out into a complete song. "I whistled it for him. His engineer was recording it, and it went into the pool of ingredients. Kanye was just collecting things. We weren't going to sit down and write a song so much as talk and spark ideas off each other. It was only when I got this song ["All Day"], the Rihanna record ["FourFiveSeconds"] and "Only One," the three tracks we did, that I went, ‘I get it. He's taken my little whistle-y thing.' It returned to me as an urban hip-hop riff. I love that record."

Though it's far from his rock roots, McCartney revealed that he's recently exposed himself to new hip hop artists. "I listen to it for, you could call it, education. I hear a lot of it and go to concerts occasionally. I went to see Jay Z and Kanye when they toured. I've seen Drake live. It's the music of now."

But he's less sure about claims that West is a genius. "I don't throw that word around [laughs]. I think he's a great artist. Take My Beautiful Dark Twisted Fantasy. I played it when I was cooking, and it was like, ‘This is good. There's some really innovative stuff.'"

During the wide-ranging interview, McCartney also addressed rumors of more unreleased Beatles songs in the vaults – notably the band's sprawling 14-minute experimental work, "Carnival of Light," which fans have been clamoring to hear since it was unveiled only once at an event in 1967. While McCartney admits that some material remains unheard, he isn't sure anyone would really want to hear it.

"That's the question: Is it worthwhile? The thing about the Beatles – they were a damn hot little band. No matter what you hear, even stuff that we thought was really bad – it doesn't sound so bad now. Because it's the Beatles."

He even floated the idea of releasing the session tapes for each Beatles' album as a box set, to see how the songs were constructed from the ground up – and give a fly-on-the-wall perspective of their working relationship in the studio.

"The talk between the takes – I've always loved that," he says. "We always had this two-track tape recorder running in case we came up with a little jam. ‘Take 36, what was that like?" But it was actually a chronicle of our dialogue. There's one bit I particularly liked: We were doing ‘I Saw Her Standing There.' I went, ‘I can't do it. I haven't got my plec.' We didn't call them guitar picks, we called them plectrums. John [Lennon] said, ‘Where is it?' – this in our thick Liverpool accents. ‘I think I left it in my suitcase.' John goes, ‘Ah, soft ass.' ‘Soft ass? I'll give you a soft ass.'"

McCartney is currently on his One on One world tour, which will end in Indio, California, on Oct. 15.