As Boulez wrote of his second book of Structures for two pianos (1961): "Does a musical work have to be considered as a formal construction with a firmly fixed direction? Could one not try to regard it as a fantastic succesion in which the "stories" have no rigid relationship, no fixed order?"

The remaining difficulty was that, as far as the naïve listener is concerned, an aleatory work does have a fixed order: he cannot recieve the impression of formal mobility from a single performance, still less with a gramophone record. This is where any analogy with, for instance, the mobiles of Calder breaks down, for music is a temporal art. Limited freedom may be evident in the flexibility and spontaneity of a performance, but not formal mobility.

—Paul Griffiths, Modern Music: a Concise History