The Philadelphia Chamber Music Society has never suffered from an ambition deficit, unless you think it's somehow not enough to simply present 60 or so extremely meaningful encounters per year with high-grade string quartets, lieder artists, and pianists.

Still, PCMS has focused its imagination a bit with its Departure & Discovery festival, which opened Thursday night at the Perelman Theater. The series within a series, whose next concert is March 6, examines the repertoire for characteristics of "late style" -- the idea that composers had something special to say late in life.

Can listeners actually discern what makes a late piece late style? Confirmation-bias theory tells us that people sort through new evidence looking to confirm existing beliefs, and this is no less true in music than in life. Still, Thursday's four pieces did a nice job of illustrating at least a couple of running themes -- that some composers were able to find a concentrated essence late in life, while others were veering off into a new language.

Beethoven did both -- and simultaneously in the Opus 111 Piano Sonata in C Minor, his last. The Brentano String Quartet grounded listeners in the musical building block known as Bach, performing the Contrapunctus Nos. I, III, V, and X from The Art of the Fugue in an impressive show of minimal vibrato and maximal expressivity. György Kurtág was generally a composer of few notes, but in his Játékok, Volume VII (games) for piano, played with great attention to detail by Jonathan Biss, he reduced music to its most essential ideas and gestures. Britten's String Quartet No. 3 was among his last works, and it catches the composer still experimenting, finding stunningly original colors and effects, vividly realized here by the Brentano.

It was heartening to hear Biss, one of the creative forces behind this festival, playing Beethoven's Opus 111 in slightly uncharacteristic form, taking more chances than usual in pursuit of an interpretive truth. The first movement drew blood. Biss not only emphasized its unsettled character, but he himself seemed unsettled, as well.

And, in a way, the second movement was no less unhinged. Biss maintained the feeling that this music could go anywhere at any point, despite being tethered to a theme. In the fantastical final minutes, Biss raised the music from beating to fluttering to a euphoric manic surge that could only be described as pure spirit. After this, there is nothing more to be said, and Biss wisely chose not to say it. There was no encore.