Why, in the case of Messrs Robert Plant and Jimmy Page’s Led Zeppelin hit, Mr Mark Ronson’s “Uptown Special” and other borrowed songs, the lines are very much blurred .

It now seems likely that Mr Pablo Picasso never actually uttered the words “Good artists copy, great artists steal.” Whatever their provenance, how hollow those words will now sound to Mr Robert Plant and Mr Jimmy Page, as they prepare for trial in Los Angeles over the authorship of “Stairway To Heaven”. The song, which featured on Led Zeppelin’s fourth album, released in 1971, has earned its writers millions of pounds in royalties over the past 45 years – money that, in an echo of last year’s court ruling over who deserved credit for Mr Robin Thicke’s number-one single “Blurred Lines” – may end having to be shared with the splendidly named Mr Randy California of the American band Spirit, whose 1968 song “Taurus” is at the centre of the current dispute.

The $7.4m awarded in damages to the family of the late Mr Marvin Gaye in the “Blurred Lines” case may have hit Mr Robin Thicke and Mr Pharrell Williams hard in the pocket, but it is the chill wind that still blows in the aftermath of the trial that should worry their fellow musicians more. Ask most prominent songwriters and they’ll admit to a sudden moment of panic in the first minutes after arriving at what their instinct tells them sounds like a hit. Sir Paul McCartney, waking up with the melody of “Yesterday” in his head, was so convinced he’d heard it before, he played the song to everyone he encountered and asked them if it sounded familiar. Songwriters have long been cautious about this decidedly grey area. What has changed since “Blurred Lines” is that they are now terrified about it. Mr Sam Smith, alerted to the similarities between his single “Stay With Me” and Mr Tom Petty’s 1989 hit “I Won’t Back Down”, hastily amended the songwriting credits (and the royalty split). An even more glaring example of the new reign of terror occurred with “Uptown Funk”. Two months after the “Blurred Lines” ruling, Mr Mark Ronson quietly ceded the five writers of The Gap Band’s remarkably similar sounding “Oops, Up Side Your Head” a share.

Yet, with the exception of the “Blurred Lines” case (which set a potentially ruinous precedent by ruling that Messrs Thicke and Williams had purloined the “vibe”, rather than the melody or lyrics, of Mr Gaye’s song “Got To Give It Up”), all of these cases are merely retreads of previous legal battles and out-of-court settlements. Mr George Harrison’s 1970 song “My Sweet Lord” didn’t so much borrow from The Chiffons’ hit “He’s So Fine” as wrestle it to the ground and run off with the body parts, sparking a protracted court battle that Harrison eventually (and inevitably) lost. Oasis’s second single “Shakermaker” half-inched The New Seekers’ “I’d Like To Teach The World To Sing” with similar brazenness, and Mr Noel Gallagher was punished accordingly. Radiohead may have been more subtle on “Creep”, cosying up to The Hollies’ “The Air That I Breathe” rather than giving it the blunt-force treatment, but the result was the same: the writers of the latter song now share in the proceeds.

In the old, pre-“Blurred Lines” days, many of these judgments came down to intention. As most musicians will confirm, they rarely steal deliberately; it is more a case of there simply being too much music in the ether for the instinctively absorbent ears of songwriters to ignore. Whether the Houston rapper Just Brittany agrees with such a charitable interpretation remains to be seen. Last year, she accused Rihanna of ripping off her 2014 song “Betta Have Money” on the Bajan’s hit single “Bitch Better Have My Money” – and the titles alone do rather get you thinking. In this murkiest of areas, you either go to court, keep shtoom, or take some dosh under the counter. But the inference is clear. Be afraid. Be very afraid. For the (blurred) battle lines have been drawn. Mr California will see you now.