Earlier this autumn I spent a few weeks reading about the German director Werner Schroeter in order to write about Facets’s retrospective of his film work. It was probably the longest period of time I’d ever spent thinking about performance art, and I came out of it feeling barely capable of writing about it competently. Last week I was reminded again of how little I knew when I reviewed the documentary Marina Abramovic: The Artist Is Present, which also concerns the medium. Performance art seems to operate in a unique aesthetic language; and though it’s technically more immediate than any other art form (there’s no canvas, script, or musical instruments separating artist from audience), it can be the hardest to assess. At one point The Artist Is Present recounts a piece made by Abramovic and her former partner Uwe Laysiepen called The Lovers, in which each one walked across the Great Wall of China starting from opposite ends; the moment at which they passed marked the end of their 12-year romantic and creative partnership. By what criteria do you determine the value of that sort of thing?