a surprisingly difficult question (If you are dealing with patents people you are harshly confronted with it)

Yes indeed. As witness suits regarding copyright on pieces of music. If someone nicks a brilliant, catchy chorus tune from another song, but has a totally different verse tune, are they infringing copyright? How important is a similarity in lyrics? How important a similarity in the instruments used, or harmonies?

It's rare that this doesn't all end up incredibly subjective.

I can hear a new song on the radio and clearly recognise similarities to songs I've heard before. This may be a good thing (as it provides more of what I like) or a bad thing (as the song "lacks originality"). But nothing is ever totally original; or everything is original, depending upon how you're looking at it.