“It is very true that one cannot always go by the principles of Marxism in deciding whether to reject or to accept a work of art. A work of art should, in the first place, be judged by its own law, that is, by the law of art. But Marxism alone can explain why and how a given tendency in art has originated in a given period of history; in other words, who it was who made a demand for such an artistic form and not for another, and why.”