The relative roles of innovation & continuity are a central topic of debate surrounding contemporary music, and to some extent have been central issues throughout the history of art music. In order to be "classical," continuity must be present in some sense, and although innovation per se need not be present in every work, it is equally true that a new work cannot be exactly like an old one. What frequently distinguishes masterpieces from more ordinary music is an ability to transcend such a simple duality. Machaut's output is among the most transcendent in this sense, and so serves well to orient a discussion of the broader topic. Machaut's output as a whole reinforces the impact of his individual works, both by placing them into a more sophisticated context and by providing an overarching framework which serves to connect them to earlier & later compositional styles. Machaut's innovations must be seen therefore as inherent to establishing the continuity of his oeuvre, whether by strengthening connections between genres or by suggesting implications which drove subsequent music history.
When discussing Machaut's poetry, it is already evident that his breadth of style was enormous. The effect of composing in an old-fashioned and poorly preserved form such as the lai in juxtaposition with emerging genres such as the rondeau & ballade cannot be underestimated, even if the differences between them seem less evident to us today. The composers who provide some precedent for Machaut, Jehannot de l'Escurel & Adam de la Halle, include only partial music for their few extended narrative works, and indeed it has been suggested that an important portion of that music consists of quotes from traditional tunes. It is almost impossible to appreciate Machaut's lais due to the lack of surviving precedent, but that he did not neglect such a genre is suggestive of his personality, as is his basic technical redefinition of it. The distinction between secular verse forms, centered on either traditional narrative or courtly songs of troubadours & trouvères, and more technical polyphonic styles such as the motet was still rather severe until shortly before Machaut's time. Individual composers who are known to have written both polyphonic & monophonic works comprise a short list, and other than a few pregnant side-by-side appearances such as in the Roman de Fauvel, the idioms were almost entirely separate. Even in the relatively extensive catalog of Adam de la Halle, the secular polyphonic works are short and mostly in note-against-note style, as opposed to the more technical orientation of the Petronian motet. Machaut's polyphonic songs represent an amazing synthesis on this basis, not only on account of their innovations, but also due to the previously separate lyrics threads which are developed and combined.
Much can be made of Machaut's use of vertical intervals in polyphonic music, as he often seems to follow some conventions of the Ars Antiqua rather than entirely those of the early Ars Nova. One possible conclusion is that whereas the originators of the Ars Nova sought to differentiate their vertical style from earlier music, Machaut felt no such need. Ars Nova notation was primarily a scholastic development centering on the Latin motet, serving to bring to conclusion a series of steps taken to refine rhythmic divisions. Embracing it so enthusiastically for lyric poetry in the first place may have been linked directly to Machaut's taste for polyphonic songs, but in any event the simultaneity of these two developments is much of what makes Machaut's output so singular. Much as Machaut seems to have felt no need to differentiate his vertical style from Ars Antiqua music, he seems to have adopted the improved rhythmic prescription of the Ars Nova notation as a means toward a more nuanced notation of older rhythmic patterns as much as for any consciously novel features allowed for the first time. In this we can see that Machaut was a very practical, apolitical musician. So much so, in fact, that others (specifically musicologist Emmy te Nijenhuis) have noted similarities between Machaut's style of rhythmic variation of melody and that of one of my other principal interests, Carnatic music (in neraval). There is no implication here that Machaut borrowed from Indian musicians or vice versa, unless the ideas are simply reflective of a murky Indo-European bardic tradition or even the patterns of the human mind, but the sheer practicality and musicality of Machaut's concerns are underlined by this correspondence. Machaut's approach to harmony, however, remains resoundingly his own.
As much as Machaut's oeuvre seems to have arrived on a tense musical scene with sharp battle lines, and essentially subsumed those political debates without confronting them, i.e. as much as it seems to have fallen under the radar of the critics of the time, it established itself as central to the course of French (and to some extent Italian) musical practice by his old age. Machaut's musical style changes through time, fading seamlessly into the Ars Subtilior of the next generation, a generation with an array of more or less distinguished composers writing in a musical idiom similar to Machaut's late works. A comparison with Beethoven is inevitable, and it is really the extent to which he shaped future music in such a continuous fashion that illuminates the continuity involved in Machaut's early synthesis of style. Nowhere is such a retrospective view so prominent as in appraisals of the Messe de Nostre Dame, and indeed a discussion of his achievements in secular song neglects his work in Latin polyphony, whether mass or motet. His modest output in the motet form shows close continuity with the origins of the Ars Nova, along with his own personal sense of melody & harmony, but his mass cycle was largely unprecedented. Since other less unified Mass Ordinary cycles, not to mention various individual sections, had been set to polyphony, it is doubtful that Machaut's cycle was seen as revolutionary. It may well have been seen as vain. Moreover, the genre did not flower fully until well after Machaut's time, so that what we now see as his greatest innovation, the unified cyclic mass, had less forward continuity than most of his output.
In some sense, an innovation needs forward continuity, a legion of imitators, before it is really an innovation instead of a curiosity. Absent this, is there any value to prescience per se? I think not, yet such works can be appealing on their own terms, as is certainly the case with Machaut's Mass. The cyclic mass was in many ways an inevitable development, yet one carried out by Machaut with high musicality. Do we know any bad music by Machaut? He has no "Wellington's Victory" on his resumé. The continuity within his output is impressive on this basis, and indeed the mass movements use techniques from other genres. Innovation & continuity are not easily separated in Machaut's output, suggesting not only that he was an epochal composer, but that those concepts cannot be set so easily in opposition when developing a mainstream tradition today. Indeed, the two aspects can be viewed differently from different perspectives, and not only in the sense of an innovation seeming continuous in retrospect. The mass seems more innovatory in retrospect. I have preferred the "fluidity & resonance" pairing in the Machaut series, which may be more confusing but ultimately describes better how musical changes and milestones are perceived over time.