A bad artist who doesn't know any better will point to Picasso, or Monet, or van Gogh, to defend his bad art. You can't limit art with your narrow definitions, he'll say.
It's true; great art defies rules. This is because art is so richly complex a concept that any clean and rigorous definitions we try to come up with for it will be hopelessly naive. We're not smart enough to correctly draw boundaries for art. Every time somebody has set rules, a great artist has come along and broken them.
So we cannot set rules on art, yes?
Here's the problem: bad artists can break rules, too.
The reason we try at all to set rules is so that we can tell good stuff from crap. We're pretty sure there is a difference between good stuff and crap. So we try to decide what that difference is. When we've come up with an answer, though, it's always been wrong, or at least shortsighted.
But just because we can't codify our standards doesn't mean we should pretend we don't have any.
What do do?
Art isn't alone in this. There are many kinds of endeavor which fall into a problematic category: an endeavor that almost definitely has value, and ought to be undertaken, but the practice of which either cannot or should not be quantitatively evaluated regarding its value. Art falls into this category. So does education. So does philosophy. So do many things.
With any of these things, we end up, at any particular point in history, either trying to constrain it with ridiculous rules, or allowing it to produce huge quantities of utter crap. Sometimes we get creative and manage to do both at the same time.
I have a daydream that there exists a possible solution to this dilemma. I keep trying to figure out what it is.
I'm probably being shortsighted.
But it would be nice if we, as a civilization, knew how to talk frankly about the difference between breaking the rules because the rules are wrong, and breaking the rules because you suck.