Sunday's opening of the 24th season was emblematic of what this group is about. The tickets are still cheap (usually $23), which the society manages, in part, by being smart about where it holds concerts. It is officially a resident company of the Kimmel Center, but this year uses nine venues. That Claude Frank's Sunday recital was booked in the 300-seat Benjamin Franklin Hall of the American Philosophical Society spoke to the fact that the society gravitates not toward big careers, but ones whose substance has built a modest and passionate following.

While the series consists largely of imports, musicians form a true society, often playing together over the summer at Marlboro Music (a related organization) and in other chamber configurations.

Frank has been a member of this club more or less since Marlboro's start in the 1950s. At 83, the German-born, New York-trained Frank seems diminished. He never was a showy technician, and he continues to explore ideas rather than flaunt facility. This, too, is related to the artistic ideals of the group that presented soprano Hildegard Behrens a few years ago at age 66 in a vocal form that, though far from tip-top, was frighteningly powerful in other ways.

Frank is a different kind of artist, but also highly individualistic. He argued, while often struggling for notes, that to admire the kind of great technique common today is to miss more important matters.

His supreme legato delivered Mozart's Piano Sonata in C major (K. 330) from the realm of the amateur to sage status. I'd trade any pianist's last few bars of the singing second-movement "andante" for Frank's poetry, giving the thematic material a closing both hushed and majestic.

Either of the two other works on the program could have reduced a lesser pianist to tears, and at one point in Schubert's Sonata in B flat major (D. 960), Frank did seem stretched by the sprawling scale and physical demands. Certain notes didn't speak. Some passages sounded slowed to accommodate the pianist's limitations. But it wasn't just the thrill of triumph over frailty that made this worthy. Frank is a canny master of expressivity, cradling phrases in unusual ways and endowing the keyboard with an unfailingly refined sound.

Making sense where none is obvious was Frank's most magnificent gift to Beethoven's Piano Sonata in C minor (Op. 111), one of those late Beethoven works whose blithe originality constitutes a series of unanswerable questions. Again, technical concerns dissolved amid an interpretive statement that didn't provide answers so much as provoke new ones. Rare ability, that, and in Frank, is easy to cherish.

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