Clive Bell in his classic essay "Art" states that only one thing can distinguish art from what is not art: "significant form." Bell wrote:

"There must be some one quality without which a work of art cannot exist; possessing which, in the least degree, no work is altogether worthless. What is this quality? What quality is shared by all objects that provoke our aesthetic emotions? What quality is common to Sta. Sophia and the windows at Chartres, Mexican sculpture, a Persian bowl, Chinese carpets, Giotto's frescoes at Padua, and the masterpieces of Poussin, Piero della Francesca, and Cezanne? Only one answer seems possible - significant form. In each, lines and colors combined in a particular way, certain forms and relations of forms, stir our aesthetic emotions."

Bell was one of the founders of the formalist theory of art. In his work Art he claimed that representation and emotion in themselves do not contribute to the aesthetic experience of a painting. Instead it is the significant form within the painting which determines its artistic content.

He defines Significant Form for painting as "relations and combinations of lines and colours" and considered it to be common to all works of visual art. He went on to use significant form as a definition of all art. His theory relies on treating "aesthetic experience" as an emotion distinct from other emotions, and one that is triggered by significant form - the common quality of any work of art.

source: wikipedia online encyclopedia