Paul McCartney got out of bed one morning in late 1964 and sat down at the piano, driven to play the melody that was pouring out of him.  It seemed so perfect, so fully formed, that the experience totally freaked him out.  He said, “Wow, what is that? I must have heard it somewhere else before.”  For weeks, he played it relentlessly for his bandmates, for producers, for his music-minded father, for his friends and colleagues, asking if they recognized it from somewhere.

They all agreed:  Nope, that’s a new song.  Nice job, Paul.  And the standard “Yesterday” was born.

Even songwriters are a little spooked by what happens when a song is created.  They can’t explain it.  The muse strikes.  An intriguing chord progression, a compelling rhythm, a memorable melody, a bit of lyric.  But many will tell you that they often wonder: Did I really just come up with this?  Is this really new?  Or did I hear this somewhere before?

Popular music has long been criticized as derivative.  But as John Lennon once famously said, “Well, you know, there are only so many notes to work with.”  True enough, but there are so many more combinations and permutations of chords, melody lines, tempos, arrangements and production techniques that can make near-identical songs on paper become radically different treatments on records.

So the question is: When is a song original, and when is it a copy (intentional or not) of something that already exists?

There are many examples of songs that sound like obvious ripoffs of something else.  Many of these go by unnoticed, especially when the works in question are obscure.  But if a new release sounds a lot like someone else’s former hit, then often a challenge is made, and credit and/or money is rewarded in out-of-court settlements.  Particularly nasty disputes end up in contentious court actions, where lawyers must prove to a jury that the artist knew of the earlier work, and that the new song too closely resembles the old one.

The latest version of this phenomenon involves “Stay With Me,” the song that won the Grammy last week for Song of the Year.  While the Record of the Year award (which “Stay With Me” also won) goes to the performing artist and the producer of the recording, Song of the Year is awarded to the composer.  Taking the stage to collect the trophy were Smith and his two co-writer colleagues, James Napier and William Phillips.

But wait a minute, not so fast.  There are those who detect an uncanny resemblance between the chorus of “Stay With Me” and the chorus of Tom Petty and the Heartbreaker’s 1989 hit, “I Won’t Back Down,” penned by Petty and producer Jeff Lynne.  Petty’s publisher approached Smith’s people about this several months ago, and they conceded that although Smith and company were unfamiliar with Petty’s song, it did indeed sound similar.  A 12.5% stake in royalties was offered, and accepted, and the matter was settled amicably, according to all concerned.  Petty’s and Lynne’s names are now officially listed as co-writers of “Stay With Me.”

Petty’s fans groused that he and Lynne should have been up on stage with Smith Sunday night, or at least mentioned in Smith’s acceptance remarks.  On the other hand, those who don’t hear the resemblance between the two songs criticized Petty for being, well, petty.

Petty himself has said matter-of-factly, “It’s happened to me. I think every songwriter must have had that problem from time to time.  You’re working on a song and then you realize you’re channeling a melody from somewhere else.  If I just change a note or two, it’s still going to be in my head that it’s that other song.  I usually just pitch it, and start over.”

Smith, all of 22 years old, said, “It’s a very regular thing that happens, and I didn’t realize that. I’m still learning about all this stuff.”

Indeed, it is a learning process for any aspiring songwriter.  As veteran Los Angeles composer-producer Marc Tanner puts it, “Sometimes you’ll write a whole song and think it’s amazing and new, and then your publisher says, ‘It’s too close to that Arctic Monkeys song! Change it!’  And you say, ‘I’ve never even heard of them! How can I steal it?!’  But ignorance, not knowing a song, is no excuse.  It’s all about who wrote it first and copyrighted it.”

The late great George Harrison learned this painfully. In 1970, after years of operating in the long shadows of Lennon-McCartney, Harrison released his landmark solo album, “All Things Must Pass,” with its flagship single “My Sweet Lord,” a phenomenally popular #1 single in more than a dozen countries.  It didn’t take 60 days before a modest fellow named Ronald Mack filed a lawsuit against Harrison, claiming that “My Sweet Lord” was a blatant ripoff of his 1963 composition “He’s So Fine,” a #3 hit for the girl group The Chiffons.

This was not exactly big news.  People in the industry rolled their eyes, knowing these kinds of claims are almost always dismissed or settled out of court for a few bucks.  The rest of us didn’t know much about it, and we didn’t much care.  Harrison, meanwhile, listened to the comparison and thought, “Whoa, maybe I did accidentally copy it.”  His lawyers offered to settle for a very generous 40% of royalties, but the offer was refused. In 1976, a court ruled that Harrison was guilty of “subconscious copying” of the earlier Mack song.  Because he admitted to knowing “He’s So Fine,” it cost him a boatload more in royalty money, and worse, it had a lasting effect on his career.  As he admitted later, he found himself paranoid every time he tried to write a song, thinking he might be channeling some previous work.  “Let’s face it,” he said, “99% of popular music is reminiscent of something else.”

Indeed.  Robin Thicke and Pharrell Williams are currently being sued by the estate of Marvin Gaye, who claim that the pair’s 2013 smash “Blurred Lines” borrows far too liberally from Gaye’s 1977 hit “Got to Give It Up.”  Were Thicke and Williams familiar with Gaye’s song?   And if so, can it be proven they intentionally (or unintentionally) copied it?  A jury is now hearing arguments.

Seventies hard rock icons Led Zeppelin had a notorious reputation for blatantly lifting guitar riffs from old blues tunes and folk songs as the foundation for album tracks, almost defiantly goading the authors to prove it.  Two tracks from their debut LP in 1969 have been successfully challenged: “Babe I’m Gonna Leave You” was in fact written by folkie Annie Bredon and recorded by Joan Baez in 1961; and “Dazed and Confused” was in fact written and recorded by Jake Holmes in 1967.  Both composers now receive songwriting credit on those LZ recordings.  Zep’s early defining anthem, “Whole Lotta Love,” is in fact based rather obviously on blues great Willie Dixon’s “You Need Love,” and his name now appears as a co-writer on that track as well.

And then there’s this amusing example of intentional song copying:  The Four Tops had a monster #1 hit in 1965, “I Can’t Help Myself (Sugar Pie, Honey Bunch),” written by the Motown songwriting team of Brian Holland, Lamont Dozier and Eddie Holland.  Because the band’s former label had rush-released an older Four Tops song to capitalize on their success, Motown head Berry Gordy demanded a legitimate follow-up single to be released within 48 hours.  The songwriting team, pressed for time, reversed the chord pattern and used the same beat and similar melody line and brazenly called it “It’s the Same Old Song,” and the record went to #5.

I’m pretty sure you can’t plagiarize your own song… And yet…