The viewer shouldn't need to be told what art is. If he or she needs to be told, the artist has failed.But some works make no sense because the viewer has an idea of what art is.That's the reverse effect: the viewer has been taught that some things are 'art' .A pile of cans therefore can't be art to them because art isn't like that. But a pile of cans can be art. I once saw a pile of old cans which were pleasing just by the way they had been arranged. Surprised me. They'd been arranged by Warhol. But some artists, including, I suspect Warhol sometimes in later life, traded on their public name and fame as an artist and took the urinary fluid. The bloke who installed an ordinary wall light, set to go on and off at preset intervals, at the RA exhibition one year, and wanted £8,000 for it, was definitely one such!