With music, we must learn what to listen for—or, indeed, what not to listen for. After a 1964 concert in Berlin of Xenakis’s music in the 1950s, the great Nadia Boulanger ... said to the composer in her usual forthright, no-nonsense manner: “Xenakis, you don’t know how to develop your themes!” “What themes?” he replied reasonably.
CHARLES ROSEN, Freedom and the Arts, Harvard U. Press, p.237
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I wonder if Nadia Boulanger's remark was simply a joke which somehow went over Charles Rosen's head. Surely by 1964 Boulanger must have been familiar with the decades-old music of composers who "did not know how to develop their themes", e.g., Schoenberg's Erwartung (1909) or Varèse's works from the 1920s (Ameriques, Arcana, Ionisation). Or perhaps she was too shocked by what she heard and blurted out the first thing that popped into her head (thematic development may have been a subject of criticism she frequently directed at her many composition students). Either possibility sounds more plausible to me than Rosen's (admittedly funny) portrayal of Boulanger as a musical dimwit.
