Jazz is difficult to define because it encompasses a wide range of music spanning a period of over 100 years, from ragtime to the rock-infused fusion. Attempts have been made to define jazz from the perspective of other musical traditions, such as European music history or African music. But critic Joachim-Ernst Berendt argues that its terms of reference and its definition should be broader, defining jazz as a "form of art music which originated in the United States through the confrontation of the Negro with European music" and arguing that it differs from European music in that jazz has a "special relationship to time defined as 'swing. Jazz involves "a spontaneity and vitality of musical production in which improvisation plays a role" and contains a "sonority and manner of phrasing which mirror the individuality of the performing jazz musician". In the opinion of Robert Christgau, "most of us would say that inventing meaning while letting loose is the essence and promise of jazz".