Tony Godfrey, author of "Conceptual Art (Art & Ideas)" (1998), asserts that conceptual art questions the nature of art, a notion that Joseph Kosuth elevated to a definition of art itself in his seminal, early manifesto of conceptual art, "Art after Philosophy" (1969). The notion that art should examine its own nature was already a potent aspect of the influential art critic Clement Greenberg's vision of Modern art during the 1950s. With the emergence of an exclusively language-based art in the 1960s, however, conceptual artists such as Art & Language, Joseph Kosuth (who became the American editor of Art-Language), and Lawrence Weiner began a far more radical interrogation of art than was previously possible (see below). One of the first and most important things they questioned was the common assumption that the role of the artist was to create special kinds of material objects.