Morris Weitz, "The Role of Theory in Esthetics"
Weitz argues that it is not possible to develop an aesthetic theory, in the sense of a statement of the necessary and sufficient properties of art. Following Wittgenstein, he says we should ask not "what is art?," but "how is the concept of 'art' used?" Doing so, we see that 'art' is an open concept, that is, one whose conditions of application are amendable. (The opposite of an open concept is a closed one, for which necessary and sufficient conditions can be stated.) Weitz notes that sub-concepts of art like "novel," "painting," and "tragedy" are likewise open.
Why is 'art' an open concept? On the one hand, throughout history, and across the various fields of art, its use has been adapted to cover a range of things among which there can only be found "a complicated network of similarities overlapping and crisscrossing." However, as Weitz notes, 'art' is used both as a descriptive and as an honorific term. One might object that just because people have incorrectly applied 'art' to some things which were not actually art, that doesn't mean that we should see 'art' as an open concept (we may prefer to restrict its use to the sorts of art that we reserve honorifics for - in fact, we may want 'art' to solely be an honorific). But Weitz also attributes the openness of the concept of 'art' to the 'very expansive, adventurous character of art,' so that new applications of the concept can always be envisioned. Indeed, by examining the history of the arts we can see that even as an honorific, the use of 'art' has expanded in the ways Weitz indicates (cf. questions about whether To the Lighthouse or Finnegans Wake are novels, or even art).