In the fourth of his Norton lectures of 1958–59, titled "Repetition in Music" , he described a mode of composition already observable in many of his compositions since the 1920s, in which "The idea of repetition and variation can be replaced by the notion of constant rebirth, of true derivation: a stream that never comes back to its source; a stream in eternal development, like a spiral …" . A notable early example of this method is "Soli I" (1933), the first work acknowledged by the composer to have been consciously organized according to this principle. It only became a regular feature, however, beginning with "Invención I" for piano (1958), and subsequently in most of his instrumental compositions of the 1960s and 1970s: "Invención II" for string trio (1965), "Invención III" for harp (1967), "Soli II" for wind quintet (1961), "Soli III" for bassoon, trumpet, viola, timpani, and orchestra (1969), "Soli IV" for brass trio (1966), "Cinco Caprichos" for piano (1975), and the late orchestral works "Resonancias" (1964), "Elatio" (1967), "Discovery" (1969), "Clio" (1969), and "Initium" (1970–72) .