Currently there is some legal kerfuffle over who rightfully 'owns' the descending line in Stairway to Heaven by Led Zeppelin. That's because it sounds rather similar to one used in a song called 'Taurus' by 60's group, Spirit. Indeed, the estate of late Spirit guitarist, Randy California have instructed a legal team to take up the matter on their behalf to contest the issue in their favour. Jimmy Page and Robert Plant may yet be invited to a judicial court of law anytime soon! What's at stake is up to 50% of the earnings accrued from that song over an extended period of time (some 45 years).
I wonder if it's rather like a garden of roses that may look wonderful but is accompanied by the oderous smell of manure. Isn't there a stink of opportunism about this? Isn't the so-called aggreived estate and their legal bashers all intent on gaining something out of this? But then, Led Zeppelin were also in it to gain, as they rampaged their way, like marauding Vikings around parts of the world in the heady 70's. Is it not now folklore that they did a bit of plagiarising?
But how do you prove intent? Of course you can point out similarities between two 'distinct' tunes but how on earth do you prove that one was consciously influenced by the other in its creation? Let's be honest, we are all influenced by what we hear. When we go to create something, we do not start from scratch - we take what we have heard as a starting point. Are we then expected to know the back catalogue of the entire world thus making it possible to cross reference; and thus ensuring that the same twelve notes have not been arranged the same way twice? Potentially it is a ludicrous situation and frankly, if there was nothing to gain or lose then it wouldn't make a bit of difference. I suspect no one would care then.
Let me make this point: if we weren't 'influenced' by what came before then the music we would come up with as individuals would sound like rubbish. It would be like reinventing the wheel every time. But this is not how the world works - our individual lifetimes are nothing compared with the huge body of knowledge, skill etc that has been accrued by humanity at large. We cannot be expected to start from scratch; and this concept doesn't just apply to music but anything at all!
So...where to draw the line. I have no idea. Presumably a court presided over by a judge with little or no musical nounce gets to decide these matters.
I once read a music law book that began with the advice: first thing to do is to get yourself a lawyer...or a gun!'