I’ve been thinking about art recently. Here is my best attempt at a definition:
Art is a reflection of beauty found in creation which carries out or demonstrates the value or related normative implications of that aspect in the world.
So abstract art, while on the surface seems to completely lack any reference to the world, or any beauty(some might say), it actually reflects some basic structures of creation and aesthetics by its method. As much as it might try and strip itself of particulars (whether a deliberate image of an object, person, or use of many colors to render that object) it nonetheless still uses line theory or color theory and so depends on the way we are created to see. I don’t know if you have read Josef Albers, but he has a book, The Interaction of Color, that goes into huge detail about how colors look differently in different juxtaposition. Its an obvious point, but he draws it out in detail.
Graffiti, though borne by words, rarely has any reference to the words themselves. I think that’s clear from guys like ESPO, or even Cycle, whose words may or may not have normal English meanings. The words are used with only a slight reference to some real world fact (I think I remember KR having something to do with him being Korean, or is it a her?). Really, the main thing that is valued is the use of lines and structural contrast (the Germans get real excited about all this). Its almost like a topographical map, displayed publicly, and usually dynamically interacting with the space its in.