(composed in February 1969), Glass turned to a more "rigorous approach" to his "most basic minimalist technique, additive process", pieces which were followed in the same year by Music in Contrary Motion and Music in Fifths (a kind of homage to his composition teacher Nadia Boulanger, who pointed out "hidden fifths" in his works but regarded them as cardinal sins). Eventually Glass's music grew less austere, becoming more complex and dramatic, with pieces such as Music in Similar Motion (1969), and Music with Changing Parts (1970). These pieces were performed by the Philip Glass Ensemble in the Whitney Museum of American Art in 1969 and in the Solomon R. Guggenheim Museum in 1970, often encountering hostile reaction from critics, but Glass's music was also met with enthusiasm from younger artists such as Brian Eno and David Bowie (at the Royal College of Art ca. 1970). Eno described this encounter with Glass's music as one of the "most extraordinary musical experiences of [his] life", as a "viscous bath of pure, thick energy", concluding "this was actually the most detailed music I'd ever heard. It was all intricacy, exotic harmonics". In 1970, Glass returned to the theatre, composing music for the theatre group Mabou Mines, resulting in his first minimalist pieces employing voices: Red Horse Animation and Music for Voices (both 1970, and premiered at the Paula Cooper Gallery). After differences of opinion with Steve Reich in 1971, Glass formed the Philip Glass Ensemble (while Reich formed Steve Reich and Musicians), an amplified ensemble including keyboards, wind instruments (saxophones, flutes), and soprano voices. Glass's music for his ensemble culminated in the four-hour-long Music in Twelve Parts (1971–1974), which began as a single piece with twelve instrumental parts but developed into a cycle that summed up Glass's musical achievement since 1967, and even transcended it—the last part features a twelve-tone theme, sung by the soprano voice of the ensemble. "I had broken the rules of modernism and so I thought it was time to break some of my own rules", according to Glass. Though he finds the term minimalist inaccurate to describe his later work, Glass does accept this term for pieces up to and including Music in 12 Parts, excepting this last part which "was the end of minimalism" for Glass. As he pointed out: "I had worked for eight or nine years inventing a system, and now I'd written through it