Art! Where does it begin? Where does it end? And who's asking?

Okay, so Shepard Fairey's art involves defacing other people's property, sometimes with permission, sometimes without. He pastes his truly wonderful poster art on buildings, which are pieces of architecture, which is a form of art.

If he does this in Allston, people deface his posters. Tear out chunks and (since the shreds aren't scattered around the pavement, and we know nobody's ever swept that lot) keep them as souvenirs. Without permission. This changes the art. Defaces it. So is defacing a Shepard Fairey poster a form of art?

And then there are the truly un-wonderful scrawls elsewhere on the wall. Scrawled without permission. Again we ask: art? Did Fairey cover any of this crude vandalism when he slapped up his poster? Did he then deface the art that was already defacing the architecture? Only to have his art defaced? By an artist? When I take a picture of all of these things together, is that art too?

Clearly, what we learn from the above scene is that the only thing out of place on that wall is the one thing that belongs there -- the fairly artless sign for the International Bicycle Center.

How can that be?