Many people think it's wrong to significantly alter a work of art, not just because the result is aesthetically inferior, but because doing so wrongs the artist or is otherwise offensive. It's easy to see why, say, defacing a painting might be offensive. It's less obvious, though, why altering a work of music of literature might be bad. After all, a painting is a concrete, singular object; but novels and poems and symphonies are not. I can't ruin "Robinson Crusoe" or Beethoven's 5th in the way that I can a Matisse or a Van Gogh. Why should it seem problematic, for instance, to perform a piece of music in a manner deemed inauthentic, given that there's a sense in which "altering" or otherwise degrading the piece in its original, authentic form is just impossible.

Your question seems to raise two distinct kinds of issues: first, and most generally, what, if anything, is wrong about altering a work of art; second, in what respect can different kinds of works of art--such as novels or lithographs, of which there are multiple instances or exemplars--or pieces of music or plays, that are meant to be interpreted in particular ways, be altered.

You remark that what's wrong about altering a work--say, drawing a moustache on the original Mona Lisa hanging in the Louvre (as opposed to drawing a moustache on a reproduction of the Mona Lisa, as Duchamp did), or defacing Picasso's Guernica, as I believe happened when it was hanging in the Museum of Modern Art--is that "doing so wrongs the artist or is otherwise offensive." It's not clear to me that this is indeed the case. Suppose Duchamp had indeed drawn a moustache on the original Mona Lisa. Would Duchamp thereby have wronged Leonardo da Vinici? Perhaps, but only if it's possible to wrong someone who is dead. (This issue has been treated in other posts on this site.) There might well have been some sort of offense committed in altering the Mona Lisa in this way, but the nature of the offense remains to be specified. Perhaps insofar as a work like the Mona Lisa consists in its being the particular object that it is, drawing a moustache on it would change the work, and we would thereby lose the original, and so altering such a work--or, say, modifying the original manuscript of a book--would bring it about that that object no longer existed, and that might underlie what is wrong about making such an alteration.

But what about altering a copy of, say The Great Gatsby, or a lithograph or even a poster that reproduces some work of art? These are all distinct cases, that need to be treated differently depending on the nature of the copy at issue. For example, marking up a first edition of The Great Gatsby--of which there are very few instances--could cause the value of that copy to go down, although this wouldn't change the nature of The Great Gatsby itself as a work of art in the same way that drawing a moustache on the Mona Lisa would change that work, given that numerous unaltered copies of The Great Gatsby continue to exist. (But suppose that the altered copy of The Great Gatsby were the only copy of that work remaining in the world, in any form: would writing on it change the nature of the work? It's not clear to me that it would: I'm inclined to think this is the case because I'm not inclined to identify the work, The Great Gatsby, with any particular instantiation of it. But this is a contentious position that would need further justification.) A lithograph--especially, say, a signed lithograph, or one that is part of a series, is a different matter, since such a work of art has more in common with the Mona Lisa, to my mind, insofar as part of its artistic value consists in its being the particular object that it is, than it does with a copy of The Great Gatsby. As for a poster, since its aesthetic value consists in its more-or-less faithfully reproducing the original, so changing the poster would result in its no longer representing that original.

Plays and pieces of music are a somewhat different matter. Here we must distinguish between altering a copy of a play or a piece of music, altering the original manuscript of a play or a piece of music, and performing a play or a piece of music in a way deemed 'inauthentic'. I'll focus on the last case, since that's the one on which you focus, and concentrate on music, although similar considerations, I think, apply mutatis mutandis, to plays. The question of the importance of 'period' performances has risen in prominence with the rise of the 'early music movement', which seeks to perform pieces with the kinds of instruments, and the same size musical groups, as works would have been performed when they were written. Although such performances do more accurately present the piece as it would have been heard at the time when it was written, it's not clear to me, however, that performing the same piece on modern instruments, and/or with a larger orchestra, alters the piece, since the same piece of music is being performed: the difference between the performances is a difference in the presentation or interpretation of the piece, and so does not constitute an alteration of the piece (assuming, of course, that the composer did not specify exactly how many instruments and of what type should perform the piece).

If the foregoing is plausible, then we see a difference between works of art such as musical compositions and plays, that admit of interpretation, and works that do not, but instead essentially consist in being the particular work that they are, such as certain sculptures and paintings. To be sure, there are a host of intermediate cases--such as works of literature as well as artworks whose form is not specified by the artist: this suggests that the question of just what a work of art is, and what the ontology of a work of art is, is importantly indeterminate, and requires careful attention to the kind of work of art at issue.

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