There is something emblematic in Boulez's recourse to the personal in the works he began during the second half of the fifties: to the solo voice, even to song (the cantabile of the Improvisations sur Mallarmé has no precedent in Le marteau sans maítre,) to music for himself to perform (Le marteaudates from before the beginning of his concert-hall career; Pli selon pliand Figures-Doubles-Prismes developed in parallel with that career, and became repositories for practical discoveries in terms both of the orchestra and of his own performing skills as the pianist of the Third Sonata and Structures grew into the international conductor), to a personal world of brilliantly figured, static sound. The idea of the work as a model embodiment of new musical thinking -- the idea behind the first book of Structures, Le marteau, and everything Stockhausen had written from Kreuzspiel onwards, an idea that belonged with the Darmstadt notion of the composer as his juniors' examplar -- was perhaps harder for him to sustain as it became clear that there was no such thing as 'the avant garde', that the years of mutual interests were passing, and that composers were going their own ways.
Xenakis's earlier criticism had come from outside the central group, and so perhaps could be brushed off, but in 1960 Ligeti made some of the same points within the pages of Die Reihe1 citing examples from Boulez, Stockhausen and others to show how serial principles had either proved self-defeating or been replaced by 'higher order' principles, such as those governing the temporal structure of Gruppen. Out of this analysis he derived the notion of 'permeability' in music: a musical structure is said to be 'permeable' if it allows a free choice of intervals and 'impermeable' if not (he gives the example of Palestrina's music -- a symptomatic choice for a contrapuntalist -- as being strictly defined by harmonic rules and hence 'impermeable' to an unusual degree). But permeability and impermeability could also be features of texture, rather than harmony, as was the case in contemporary music. Using the example again of Gruppen, he noted how 'a dense, gelatinous, soft and sensitive material can be penetrated ad libitum by sharp, hacked splinters . . . "Soft" materials are less permeable when combined with each other, and there are places of an opaque complexity beyond compare' -- beyond compare, that is, in late 1958, when Ligeti wrote the article. By the next year he had completed his orchestral piece Apparitions, whose first performance, at the 1960 festival of the____________________