Original: Tony Godfrey, author of "Conceptual Art" (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).
Simplified: Conceptual art even questions the nature of art. Joseph Kosuth elevated this into a definition of art itself in his early manifesto of conceptual art.