According to Jessa Crispin
In a recent interview about Jane Austen, Fran Lebowitz said that great art is “not a mirror, it’s a door.” Mediocre art is a mirror, and either you get it or you don’t, either you relate to it or you don’t.
According to George Bernard Shaw
You use a glass mirror to see your face; you use works of art to see your soul.
Aside: According to Wikipedia, the title of Aldous Huxley’s mescaline book The Doors of Perception comes from a quote from William Blake
If the doors of perception were cleansed every thing would appear to man as it is, infinite. For man has closed himself up, till he sees all things through narrow chinks of his cavern.
Speaking of George Bernard Shaw, he also said
Better keep yourself clean and bright; you are the window through which you must see the world.