Banksy

one doesn't need to define art, per se.
the more that one trie to delineate it into a tradition or school, the more later artists will simply tear those walls down. do you think that dan flavin is art? is richard serra? both those artists create work that has a fairly specific function.
but even go back to your foundation course. is the last supper art? venus de milo? look at almost any renaissance high art painting or sculpture and you will find work that was created with the specific purpose of giving glory to the almighty.
besides, claiming that banksy's work is denied it's status as artwork simply because it creates a name for him is also useless. durer is perhaps the best known artist in the field of early printmaking and yet he also pioneered the name or brand of the artist.
and then how do you define function? something that does something? anything?