This article analyzes the production of certain ideas in musicology, concentrating the focal on the Essay of Célestin Deliège on Complexity. It crosses surrounding areas of the ideology of Edgar Morin and Raymond Boudon. The musical current of New Complexity, at the end of the XXth century, is presented, with its most known composer, Brian Ferneyhough. To understand the thought of Deliège, the other papers allow to encircle the “good reasons” (words of the sociologist Boudon) to define the Modernity, through what Nicolas Darbon calls his dodécalogue. The musical Modernity is a contemporary idéo-myth (words of the sociologist Morin). The underlying philosophic system is the one of Theodor W. Adorno. It is interesting to observe the progress of an original idea towards its interpretation, through the notion of “tour de force” (Adorno, Ästhetische Theorie), which Deliège perceives in the hiatus between virtuosity and aesthetic result. It is interesting to understand the Deliège’s notion of “musicologie critique” - between sociology and politics. The article studies the “effect of position”, “effect of communication”, “epistemologic effect”. Deliège embodies the paradox of the musicologist, between science and doctrine, enters rationnalism and lucidity. Maybe, he realizes the correctness of the criticism according to Baudelaire : partial, fascinated and political?