It should include a clause somewhere that reflects the timelessness of art; that is, if something was found to have artistic merit at one time, then it can always be considered "art" because it's a reflection on the aesthetics of that culture.
This assumes that every future culture will respect the aesthetics of every previous culture. If in the future everyone decides that what we consider art to be totally without any artistic merit, I see no reason to believe that they'd call it art simply because art is supposed to be timeless.
I don't see any reason to believe that. First of all, that's an incredibly far-fetched situation; nobody's ever decided that all of a particular generation's art was worthless and burned it, and I see no reason why that might ever happen. Even if it did, we still wouldn't say "this is no longer art". It doesn't make a whole lot of sense to say "this was art once, but now everyone thinks it's shitty so it's not art anymore".
You've got it backwards. Stuff gets put in museums because it is art; it doesn't become art when you put it in a museum.
Just like how awards are only given to good musicians?
No, not really. We're trying to differentiate between art and non-art here, not good art and bad art. Art is put in museums because people consider it to be art. I don't know what's hard to understand about that.
That's just the thing though! Plenty of people think that art is subjective, and that they alone can decide whether something is art based on whether it suits them. It's not; although all art may not move you personally, there are still objective bases for determining whether something is art.
The only thing I think you can "objectively" look at is the number of people who feel that a given work is art or not. IMO, that's not the same as objectively determining that it is art.
On the contrary, that's exactly what it is!
Let me explain. A perhaps less roundabout definition of art would be "something which has artistic merit". However, this definition is very circular and nonintuitive. We also run into the problem of subjectivity. While everybody has a sense of aesthetics, our aesthetic-ometers are all tuned a bit differently; not everybody agrees that certain things have aesthetic appeal. That means that if we define art this way, a work may be art when it's in front of a person that likes it, and not art when in front of a person that doesn't, and that's a paradox we want to avoid.
So, we're forced to measure artistic merit in objective terms, which is rather impossible. To solve this tricky problem, we're going to employ an
intent and perception model, which you can see in my definition above. In order for something to be art, the producer has to consider it art, and an audience needs to agree that it has artistic merit. Problem solved!