Modesty may be the greatest legacy of the last century of performance. After a long and pompous era of egomaniacal instrumentalists and monomaniacal conductors, a new breed of artist--the modest purist--has emerged. The Norwegian pianist Leif Ove Andsnes is from that mold, and it's refreshing that he has managed to build a substantial career without compromising his high standards.

The Washington Performing Arts Society has presented Andsnes in the Kennedy Center Terrace Theatre in past seasons, but on Saturday afternoon it bumped him up to the much larger Concert Hall. It was both an honor and a challenge for the young artist. The WPAS further dignified the event by commissioning a new work for Andsnes, a brief but dense set of five miniatures by the English composer Mark Anthony Turnage.

Andsnes doesn't conform to the American conservatory model of pianists. In interviews, he has described an idyllic childhood, spent on an isolated island off Bergen. He matured independent of the competitiveness that marks most modern conservatories. And after building a substantial career in which chamber music, solo playing and orchestral work all had equal importance, he received the 1998 Gilmore Artist Award, a "genius" grant for pianists that has traditionally recognized players with substantial intellect over flashy young upstarts.

On Saturday, Andsnes anchored his program with two hefty romantic sonatas, Schubert's late Sonata in A, D. 959, and Schumann's muscular fantasy the Sonata No. 1 in F-sharp Minor. Both explode the classically taut form into something between a novel and a ramble. The Schubert is so experimental that it can leave audiences wondering whether the composer really meant what he wrote. It starts and stops, comes full circle for long repetitions, seems about to end yet starts over for yet more exploration and development. Its surface level is vintage Schubert--tender and lulling, melodic and wistful--but its larger form is beautifully aberrant, shockingly unbridled.

Andsnes disappeared into the performance. It's easier to say what he didn't do than to explain why the performance worked so well. He doesn't impose interpretation on the music, he doesn't revel in tone color, he doesn't dramatize. He's neither sentimental nor overtly heroic in his playing. Virtuosity is adequate but understated. The tone isn't distinctively bright or full.

But throughout the sonata (and the Impromptu in F, D. 935, also by Schubert) Andsnes made the Concert Hall feel more like a small living room after dinner, playing intimately and intelligently, and heightening the music's dreamlike effects. Schubert's textures are often built around a repeated accompaniment figure, with aphoristic melodies arranged as if in personal dialogue with each other. Andsnes never suppressed the accompaniment figures to highlight the melodic ones; the effect was mesmerizing, as if Schubert meant for his music to be listened to in a trancelike state.

The Schumann was equally effective. Andsnes has recorded the sonata before, and it's a very fine recording. His interpretation is a natural extension of his Schubert playing, with a little more volatility and a sharper, more pungent technical approach. Again, disappearing into the music, he manages to convey its inherent expansiveness and flightiness.

The new pieces by Turnage, called "True Life Stories," are small but fascinating. The cycle begins and ends with music reminiscent of England's wonderful miniaturist Peter Warlock (a k a Philip Heseltine), with three inner movements that remind one of Webern at times. The harmonies are piquant but never aggressive; the music often moves in different spheres of activity, with simultaneous but unrelated events creating a sense of spaciousness; the textures are almost always spare and transparent. Andsnes played from the score, but played as if he has already internalized the music. Other pianists may want to search out these dense musical haiku.