Conceptual art is based on the notion that the essence of art is an idea, or concept, and may exist distinct from and in the absence of an object as its representation. It has also been called Idea art, Post-Object art, and Dematerialized art because it often assumes the form of a proposition (i.e., a document of the artist’s thinking) or a photographic document of an event. Conceptual art practices emerged at a time when the authority of the art institution and the preciousness of the unique aesthetic object were being widely challenged by artists and critics. Conceptual artists interrogated the possibilities of art-as-idea or art-as-knowledge, and to those ends explored linguistic, mathematical, and process-oriented dimensions of thought and aesthetics, as well as invisible systems, structures, and processes. Artists such as Joseph Kosuth and members of the Art & Language group wrote theoretical essays that questioned the ways in which art has conventionally acquired meaning. In some cases such texts served as the art works themselves. Other figures associated with Conceptual art include Mel Bochner, Hanne Darboven, Agnes Denes, Jan Dibbets, Hans Haacke, On Kawara, Les Levine, Sol LeWitt, and Lawrence Weiner.