Avant-garde art, he argued, arose as part of a movement of cultural critique within the bourgeoisie, and is thus linked with revolution. But it moved away from critical content, and focused instead on itself, so that increasingly painting has come to be about painting, or about nothing at all, rather than depicting something else; and similar movements have ocurred within the other arts. Thus the art which began as critical of culture is appreciated by only a few in society; and the new art of the masses, or "kitsch", is fundamentally both conservative and uncultured. Kitsch is tied to mass production, and is not "genuine culture". Greenberg explained its appeal by the ease with which it carries values extrinsic to art, as opposed to those of art for art's sake. For example, a sweet little angel carries values of sympathy, romantic prettiness, and a comforting belief in the supernatural (contrast the appearance of angels in the Bible!). Who would be so crude as to suggest that it is trite and boring as sculpture or painting?
As avant-garde art has moved away from content, it has placed more emphasis on form. It has become more "pure" or "non-objective". In this respect it has become more like music. In Greenberg's words, :
"Content is to be dissolved so completely into form that the work of art or literature cannot be reduced in whole or in part to anything not itself" ("Avant-Garde and Kitsch", p. 6, in Greenberg, Art and Culture, Boston: Beacon Press, 1961). "Picasso, Braque, Mondrian, Miro, Kandinsky, Brancusi, even Klee, Matisse and Cezanne derive their chief inspiration from the medium they work in" (ibid., p. 7).
A number of writings by and about Greenberg, and assesment of his significance as a critic, may be found at http://www.sharecom.ca/greenberg/default.html