To add to the very profound answer by photographer Giles Duley, can I just pick and choose 3 quotes by Marcel Duchamp? Why him? Because he challenged the whole idea of what art actually is:
Can works be made which are not 'of art'?
The word 'art' interests me very much. If it comes from Sanskrit, as I've heard, it signifies 'making.' Now everyone makes something, and those who make things on a canvas with a frame, they're called artists. Formerly, they were called craftsmen, a term I prefer. We're all craftsmen, in civilian or military or artistic life.
What art is, in reality, is this missing link, not the links which exist. It's not what you see that is art; art is the gap.