Closed concept  

"What I am arguing, then, is that the very expansive, adventurous character of art, its ever-present changes and novel creations, makes it logically impossible to ensure any set of defining properties. We can, of course, choose to close the concept. But to do this with "art" or "tragedy" or "portraiture," etc., is ludicrous since it forecloses on the very conditions of creativity in the arts." --"The Role of Theory in Aesthetics" (1956) by Morris Weitz

"Despite anticipations in Nietzsche, the idea of an open concept—often also referred to as an open-textured concept—was first explicitly used by Friedrich Waismann in his essay on the verifiability of empirical statements. Waismann's account is reminiscent of Wittgenstein's remarks on unbounded concepts and family resemblances, and is well, if not exhaustively, understood in the light of these. Concepts of democracy, justice, and art, as well as of music and musical work, are examples to keep in mind as we proceed with the more general discussion."--The Imaginary Museum of Musical Works (1992) by Lydia Goehr