Simplify the following text for a Grade 9 native English-speaking person.
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 .