In its own time, Ars Subtilior (or "the more subtle art") was regarded as work of absolute sophistication. It came billed last weekend on London's South Bank as medieval "avant-garde" music; music purposefully cultivated to advance the formal and notational precepts of the period and, perhaps, to idealise its composers' ability to do so: ars for ars' sake, in other words - a frightfully negligent notion in the late 20th century but a worldly, bordering on naughty, one in the time of Machaut and Dufay.
After all, the high medieval aesthetic thesis held that art should be a mirror and lens to God's creation in all its elaborate naturality, not a pander to Man's vanity, in all its appetites. So it is proper to consider Ars Subtilior in its historical context, which is high medievalism in decline. The late 14th century in western Europe was a period of governmental breakdown, brigandage, sectarianism and uprising; it saw the division of the papacy, the beginnings of the banking system, the beginning of the end of feudalism. Perhaps more significantly, it saw plague.
The quiet lutes, fiddle, recorder and soprano voice of the New London Consort did not describe a clashing world riven with disease and social and economic uncertainty. Mostly they described courtly feelings and birds. But for all that, there was amazing intensity to the Consort's delivery, partly derived, as they happily conceded, from the difficulty of the music in performance.
The difficulty of the music on the ear was altogether more interesting, however, its awkwardness arising not from amplitude, harshness or excess vigour but from extremity of delicacy. "He, tres doulz rossignol joli", "Or sus, vous dormez trop" and "Par maintes foys" were so convoluted in their curly decorations, fragmentary rhythms and sound effects (lots of nightingales, ladies, sighs and dying hearts) that after a while one heard only decorations, appreciated only Catherine Bott's control over her large ornithological range.
Similarly, some of the less accessible texts - "La douce chiere", for example, which mostly comprises verbal heraldry - were set in filigreed cages of notes that baffled and dissembled and concluded with unresolved cadences to lyric puzzles. Here, complexity exceeded this listener's powers to assimilate it.
In the end it took a beautiful serpentine lament, entitled "Helas pitie" by someone called Trebor, in which Bott exceeded herself for control in restraint, to unravel things a little and demonstrate that even if we do not have the means to rationalise this music to the full, then we do still have the means to feel it. One felt anguish and thought of ballads by Ornette Coleman.
n The Purcell Room's early music series continues to 13 June. Details: 0171-960 4242