Conceptual art holds that the idea behind a work matters more than its visual form. Emerging in the mid-1960s and cresting in the early 1970s, it pushed art toward language, documentation and systems: Sol LeWitt's 1967 declaration that "the idea becomes a machine that makes the art" is its classic formula. Artists produced instructions, maps, photographs, typed statements, time pieces and temporary installations, deliberately dematerializing the art object and its market. Joseph Kosuth staged objects, images and definitions side by side; Lawrence Weiner made sentences into works; On Kawara dated paintings; Art & Language wrote theory as practice. With Duchamp's readymades as its warrant, conceptual art turned institutions, categories and the question "what is art?" into its raw material, and its habits of documentation and critique still frame contemporary art.