Summary: A painting by abstract Dutch artist Piet Mondrian has been hanging upside down in various museums since it was first put on display 75 years ago, an art historian has found, but warned it could disintegrate if it was hung the right side up now. But is it important what way up it is? And is the art important? What about other modern abstract art, is it important, is it actually art? Mark and Pete weigh in.
