From Susan Sontag's Against Interpretation:

And it is the defense of art which gives birth to the odd vision by which something we have learned to call "form" is separated off from something we have learned to call "content," and to the well-intentioned move which makes content essential and form accessory.

Even in modern times, when most artists and critics have discarded the theory of art as representation of an outer reality in favor of the theory of art as subjective expression, the main feature of the mimetic theory persists. Whether we conceive of the work of art on the model of a picture (art as a picture of reality) or on the model of a statement (art as the statement of the artist), content still comes first. The content may have changed. It may now be less figurative, less lucidly realistic. But it is still assumed that a work of art is its content. Or, as it's usually put today, that a work of art by definition says something. ("What X is saying is. . ., " "What X is trying to say is . . .," "What X said is . . ." etc., etc.)