Why did Clement Greenberg value the flat surface of a painting?

For Clement Greenberg, the flat surface of a painting was essential because he believed that Modernist art should be self-critical, focusing exclusively on the inherent properties of its medium rather than on external elements like narrative or illusionism.12

Greenberg argued that painting must be "purified" by eliminating anything that was not fundamental to its nature. Because a painting is physically a flat surface, he viewed the acknowledgment and finessing of this flatness as the defining characteristic of the medium. By emphasizing flatness, the work became autonomous and self-sufficient, existing as a purely aesthetic experience independent of literary or anecdotal content.12

In his view, a painting that addressed its own flatness—such as those by Morris Louis or Helen Frankenthaler—exemplified this "formalist" purity. By rejecting the illusion of three-dimensional space, the artist could focus on the "formal facts" of the medium, such as color, shape, and the material texture of the canvas itself.123