The life of a touring ensemble is one in which most essential decisions are made well in advance.

What music is played, and where, at the Jerusalem Quartet's Thursday concert for the Philadelphia Chamber Music Society at the American Philosophical Society, was determined at least a year ago. And being one of the best of its kind, the string quartet played its Mozart/Bártok/Schumann concert with the commitment and artistry one has come to expect from past concerts, and its excellent Harmonia Mundi recordings. But the group's collective heart seemed to lie in the one section allowing spontaneity: the encore.

The slow movement of Debussy's String Quartet was played with mesmerizing concentration in ways that had a depth of artistry, an overall ear for sound, and a degree of personality that showed, more than what came before, who this quartet is. It's a group that cares about blend, but unlike some quartets, doesn't fall back on warm, pretty sonorities to ingratiate itself to listeners. The sound markedly lacks anything extraneous, and it can be produced with force, but never feels labored. Even in Debussy's most tightly voiced moments, the individual personalities of the members could easily be picked out - especially cellist Kyril Zlotnikov's clean upper register and violist Ori Kam's unusually full-bodied sound.

This isn't to say that the rest of the concert was particularly lacking. But any performance of Bártok's String Quartet No. 4 comes with steep challenges, technically and cognitively, that no quartet can fully meet in every performance. Although well-played, the first two movements didn't have the needed comprehension.

Schumann's String Quartet Op. 41 No. 3 was a risky choice for the program's second-half prime slot. The composer seemed inhibited by the string quartet medium, and only in this quartet, his third, did he achieve a happy medium between his innate keyboard-based spontaneity and the rigor of four string instruments. The Jerusalem Quartet gave the piece subtle textures and needed heat - mainly because it found places in the music that could accommodate such qualities.

Mozart's String Quartet in G K. 387 felt thoroughly idiomatic without going the historically informed performance route, with its solid but malleable sound, and intense, non-gimmicky soft playing, projecting an unexpected element of musical soliloquy.