He has been talking about the principle of novelty in the aesthetics of modernism, but then goes on to discuss the connection between novelty and fashion:
Fashion suffers from an inner contradiction: it must of course evoke the appearance of novelty in order to stand out against the past, but at the same time it is compelled to establish itself at once as a convention, in the very moment when it comes to the fore; therefore fashion is always as if fleeing from itself. ... The novelty of fashion is abstract. ... Its decisive feature is not its content, but the mere form of switching into something always different. Even if fashion copies something from the day before yesterday, it has no tradition, whereas, in what is truly and substantially new, tradition is always contained and transcended, even if in the form of explicit negation. [op. cit. p. 96]Let me try and fill in the blanks a bit. Dahlhaus is not the only one to mention the connection between modernism and fashion: my composition teacher in the 1970s commented that Stockhausen was always "five minutes ahead of what is fashionable". Paul Johnson in his Art: A New History published in 2003, makes the critique of modernism as a kind of fashion central to the later part of the book. Interestingly, Dahlhaus, in the section just after the passage I quoted, mentions Schoenberg as an example of what is truly new, not just fashionable, because his denial of tonality is a form of inclusion of tradition in the form of explicit negation. One of the things that I have often found odd about Schoenberg is that the rhythmic gestures often seem to be taken from tonal music, even while the harmony is denying it.
I think one of the things that Dahlhaus is saying is that fashion has no memory, while real innovation does. Beethoven in his late quartets is, even according to Stravinsky, writing music that will always be avant-garde. But at the same time all sorts of deeply-rooted traditions, from the Lydian mode to folksong-like tunelets to fugue are all used to construct a fundamentally new kind of music.
I think I would want to make a distinction between the innovation of serious artists and the fashionable ingenuity of more superficial artists. I am wary of falling into the "no true Scotsman" fallacy with the use of the words "serious" and "superficial", but I'm not sure how to avoid their use entirely. Let me take an example that is remote enough from our times to avoid any ideological entanglement. I mentioned Beethoven already. So let me contrast his later work with that of his younger contemporaries, who were more in tune with the changing tastes of the day. I am thinking of people like Ignaz Moscheles and Johann Nepomuk Hummel, the latter just eight years younger than Beethoven. Both of these gentlemen, like Beethoven, achieved early renown as piano virtuosos. This was the heyday of the development of the piano as builders refined and extended the range and dynamic power of the instrument. As a young virtuoso in Vienna, Beethoven was renowned for being able to execute trills simultaneously in three different voices--examples of which we see in a couple of his piano sonatas.
Hummel was also a big hit as a young man in Vienna and even took over Haydn's post with the Esterházy family on Haydn's retirement. Wikipedia reflects at least some of the opinions in Vienna at the time when it says:
Hummel's music took a different direction from that of Beethoven. Looking forward, Hummel stepped into modernity through pieces like his Sonata in F-sharp minor, Op. 81, and his Fantasy, Op. 18, for piano. These pieces are examples where Hummel may be seen to both challenge the classical harmonic structures and stretch the sonata form.
What Hummel was doing, of course, was just making some superficial and fashionable changes, ones without much substance. Here is the Sonata in F# minor, have a listen:
The contrast between that and one of Beethoven's late sonatas is palpable:
In his later years Beethoven veered away from the fashionable path of his contemporaries and delved deeper into the fundamentals of music--found in its traditions--for the kind of innovation that lies underneath the surface. One of the most striking examples of this is his response to publisher Diabelli's request for variations on a rather tatty waltz of his own composition. Beethoven came up with what we now call the "Diabelli Variations" in which he deconstructs the crude simplicity of the waltz down to its component atoms and builds a set of variations that challenge, not the virtuosos of his day, but the great set of variations by J. S. Bach, the Goldberg Variations, that are the Diabelli's only real rivals as monumental variations. There is nothing fashionable about Beethoven's variations, but they represent real and profound innovation. It is as if Beethoven turned fashion against itself. Here, have a listen:
Perhaps I should have titled this post "Tradition and Innovation".