The definition and evaluation of art has become especially problematic since the 20th century. Richard Wollheim distinguishes three approaches: the Realist, whereby aesthetic quality is an absolute value independent of any human view; the Objectivist,1Y0-A06 whereby it is also an absolute value, but is dependent on general human experience; and the Relativist position, whereby it is not an absolute value, but depends on, and varies with,642-373 the human experience of different humans.The arrival of Modernism in the late nineteenth century lead to a radical break in the conception of the function of art,and then again in the late twentieth century with the advent of postmodernism. Clement Greenberg's 1960 article "Modernist Painting"1Y0-A09 defines modern art as "the use of characteristic methods of a discipline to criticize the discipline itself".Greenberg originally applied this idea to the Abstract Expressionist movement and used it as a way to understand and justify flat (non-illusionistic) abstract painting: