The quality possessed by all art that distinguishes it from non-art, according to Clive Bell
In his 1914 book Art, Bell puts forth his formalist theory of aesthetics. This theory is based on the idea that certain forms conjure certain emotions; the upshot of this is that art is the combination of these evocative, emotion-producing Significant Forms.
Of course, this, like most formalist discourse, has been more or less blown out of the water in the past half-century or so.
Incidentally, Evelyn Waugh, in his book Brideshead Revisited, has one of his Oxonian characters, Sebastian, read Art. Sebastian makes one of the clearest statements of his character in relation to the theory, saying, "'Does anyone feel the same kind of emotion from a butterfly or flower that he feels for a cathedral or a picture?' Yes, I do."