(It's art day here on the secret blog. You might want to read the next post before reading this one.)
I suggest below that, for me, the difference between art and not art is whether I can come to some understanding of the work without knowing anything else about it. A work that depends upon its title or the artist's politics for its impact is commentary, not art.
What, then, are we to do with Magritte's great The Treachery Of Images (La trahison des images 1928-29). Technically, I escape the potential trap, everything that I need to know is contained within the frame: Ceci n'est pas une pipe. That's not even the title, although everyone but the most annoyingly meticulous of us refers to it by that tag. But my theory is that this is the pivotal work of 20th century art because Magritte is putting an end to representational art. The first photograph was taken in 1827, one hundred (and one) years before Magritte started to paint The Treachery of Images and, by Magritte's time, photography was a mature science and a popular hobby. There was no point any more in pursuing the visual arts in order to preserve a record of what things looked like. The Impressionists, the Surrealists, the other modernists and contemporary artists are all reactions to this loss of what had, for millenia, been one of the primary motives for painting. The Treachery of Images is the exclamation point at the end of that chapter.
As this should make clear, the full meaning of a work of art can never be divorced from its context and all works of art are, at some level, abstract. Abstract art (and cubism and whateverism) can be recognized as art today because, as Magritte argued, no painting, however faithful to the original, is the thing itself. A painting need not be a representation of some tangible thing and, even if it is, it can not only be judged based upon how much it looks like the thing it represents. This was not new in 1929. Many seemingly representational pieces were actually representing an abstract concept -- courage or liberty, for example -- even if on the surface they appeared to be accurate renderings of physical things. Some works are more accurate at rendering their subject than others, but the more successful work need not be the more nearly photographic work.
Magritte's work, for example, is art not because it is a good picture of a pipe (though it is that) but because it is a good representation of human perseverance in the face of change. Art is not art that doesn't alter when it alteration finds.
On the other hand, a toilet mounted on the wall is only a urinal.