Why is it in fact that we are very picky about certain things' authenticity, while the very notion seems completely ridiculous in relation to other things ?
Rightly or wrongly, authenticity is considered crucial with paintings and sculptures for example.
Rather less so with photos.
And is is a complete non-issue with, say, films or books - although cinema and litterature are reckognized art forms just as painting or scupture.
On the other hand, though shirt making isn't an art-form, there is apparently a clear line between a real and fake Lacoste, as between a real and a fake Rolex.
So what are we actually talking about, and why does it, or should it, matter ?
A fake Lacoste is a lesser-quality product made to look like the real thing. Same as with a fake Rolex, or a fake Louis-Vuitton bag. This isn't a problem for the buyer, as the fakes cost significantly less than the real thing; indeed, that's precisely their selling point: you get to look like the real deal for a lot cheaper. Bottom line is, the manufacturers/sellers aren't really fooling anyone, and the only party left with an axe to grind is the original brand. In this case, authenticity has less to do with "being the real deal" per se, and more to do with being a proxy for quality, and for ensuring proper consumer service.
Why are the notions of a fake or original films completely ridiculous ? For one, a film, even at its most modest, requires the imput of a fairly large number of people, so that it's difficult really to attribute its quality, or lack thereof, to any single person. For another, well the proof's in the pudding: you only have to go to see it in the cinema to be able to say whether you like it or not, whether it's good or rubbish.
This second point also goes for litterature, poetry, theatre and music. Again, the proof's in the pudding, and the author's mere name will be of absolutely no use to make up for a work's shortcomings. From another perspective, we have no qualms about voicing the opinion that a given work by an otherwise highly-regarded artist is not so brilliant, or even just trash. Insofar as enjoying these works are concerned, who wrote the Arabian Nights is of no consequence; neither is whether Homer really wrote the Illiad and the Odyssey, nor the whole debate about Shakespear's authorship of his works. Likewise, that Sussmayr completed Mozart's requiem may be of historical interest, but it won't make an iota of a difference to anyone's enjoyment of the piece, or to the price of tickets to go hear it in a concert.
Take violins. To begin with, the whole concept of authenticity (was this violing really made by Stradivari ?) is not so clear cut: first, Stradivari's instruments were in very high demand, meaning that Mr Stradivari couldn't actually make every single instrument on his lonesome from start to finish. He headed a workshop employing a number of craftsmen, some or all of whom contributed in some way to building each instrument. Second, since we're talking about wooden things made in the late 17th and early 18th century, it's quite evident a lot of them have had repairs through the ages, including replacement of certain parts. And in any case, the proof's also in the pudding : if a Strad doesn't sound good, well its merely being a Strad will be scant comfort to the accomplished player.
At the end of the day, authenticity's singular significance in painting - and perhaps a few other forms that together represent but a narrow part of the art world - appears to be a result of the combination of the following characteristics:
1. A painting is a material thing (unlike, say, a film or a piece of music), usually made by one person - meaning that authenticity is a fairly clear concept.
2. Each is unique (unlike a Lacoste shirt) and is part of group very limited in size (there are only so many Picassos) - meaning that authenticity has value (if there were ten million authentic Picassos in the world, each would be worth a lot less).
3. It doesn't serve any practical purpose (where the proof would be in the pudding), so that authenticity becomes a natural objective criterion to define, classify and evaluate the object in question
In other words, in the case of paintings, authenticity is, at heart, not a proxy for the material quality or for the degree of enjoyment given by the work. Its great importance is merely a function of the characteristics of the relevant object. More to the point, those are objects that are so few in absolute terms that rarity is no longer simply defined, in relative terms, by comparing supply and demand; instead, rarity in absolute terms invites high demand, quite possibly because human nature makes us covet that which few have.
So authenticity, which takes so much space in scholarly discourse and in the price tags of paintings, is actually completely irrelevant to art, and it's striking to consider that the material aspects and characteristics of a painting can often take such precedence over its artistic quality.