Morris Weitz claims that art is an indefinable-because-open concept. I want tentatively to suggest that he conflates indefinability and openness, overlooking in the process the possibility of a definable-yet-open conception of art.
Furthermore, Weitz -- or perhaps Wartenberg -- apparently assumes that a satisfactory definition of art must be an intensional one (a specification of the necessary and jointly sufficient conditions required of each member of the set under consideration). But Weitz's preferred extensional (in this case, ostensive) definition of art is still, obviously enough, a definition.
Even so, I remain hopeful that we might construct a satisfactory -- however inclusive and broad -- intensional definition of art. See Jeff Strayer's handout for a brief summary of the relations between necessary and sufficient conditions and any attempt to define art.