This is something I've been thinking about, and it's probably not original or fullproof, but I'd like to put it up for discussion. Bear with me as I try to explain:
Art isn't something that is, but rather something that happens. There is some sort of exchange between the piece and the viewer and that's where art is. If we could somehow represent art as a visible thing, let's say light, what we would see in a museum full of people would be lights going on and off everywhere based on the interactions between people and the pieces, and not, for example, a light on every piece. This also means that a closed museum has no art (unless maybe that which is happening based on people's recallections of those interactions).
The nature of those interactions isn't yet clear to me (what happens?) but it should be some sort of stimulation and I think they might be intellectual, emotional, spiritual or even physical. That depends solely on the individual structure of who's perceiving it, because the piece is always the same
Based on this, under the right circumstances, there may be as much art in a ceramic kitty to my grandmother as in Picasso's paintings to me. But it also means that it's not up to me to say that that ceramic piece is not art and my grandomther is entitled to not getting Picasso either, because it really isn't art to her.
This sort of stimulation can be exactly the same as what happens when we're impressed by nature for example, but art needs to be human and purposeful (even if the purpose is to be as random as possible, v. Pollock).
The art professionals and critics are there to guide people and have art exist in new ways to new people, and showing others where they're seeing art in specific works.
Does this make sense?