The idea of an or the "orginal" applies to the physical entity at the end of the artistic design process when no more work by the artist needs to be performed and the work is generally ready for presentation. Sculptures in bronze may still need to be struck into the final bronze form by the foundry. None, however are copies, they are just numbered instances. The original art then, here in this example is hardly one thing. It's the final product delivered to the foundry as well as the collection of instances which might now be delivered to the public. The same applies to photographic prints.
For photography, unless we are talking about scanning a print, the term "copy" hardly applies. The latent image when completed for that moment is the original and together with the print constitute the work of art with the final instance of the bonze or print being part of a brotherhood of instances made for public participation of that work of art. Let's just consider the photographer's job after making a latent image in some recording medium, film or digital for example.
The artist then decides to delegate printing to him/herself or some outside person. The edition is either open or not. If limited then a certain number of prints are signed and numbered. That's all one can say; "This is print 4/15 and that's that". The image ready to be printed is what is original, unless the photographer changes that at the time of printing. If that happens, then the file with the new instructions becomes the original. With such printing instructions and an artist's proof, the image file or a sheet of film, many skilled printers can deliver within the requirements of the photographer. The resulting print is not a copy, rather, what it is, a print. However, once signed and numbered, and named #4/15, it is unique. That print, 4/15 is not a copy of 1/15 nor of anything! Yes, it happens to be perhaps to our eyes, identical to the first print. However, the word "copy" does not apply. The print 4/15 is merely a new instance in the potentially endless series of prints made from the original negative or image file. All are prints. Even numbering does not imply some "birth rank" as in lithographies where the original might be degraded, as the first print numbered "1/15 might have been print order 17/17, with the first two having been artists proof and BAT copy.
On the other hand, a painting is both the original and the only instance of that. All other such paintings are indeed copies or else individual originals, variations on a theme.
Asher