Dating from J.S. Bach's last decade, "The Art of Fugue" -- the center of gravity of this week's concerts by the St. Paul Chamber Orchestra -- is a monumental, even transcendental compilation of 19 contrapuntal pieces on a single theme, rife with ingenuity. The work, apparently unfinished, has sparked endless conjecture and much romantic mythmaking since its posthumous publication in 1751.

But most scholars now agree that this music was meant to be played (not just studied), that it was conceived for a keyboard instrument (though none is specified) and that an uninterrupted performance of the complete score is, as one puts it, "an exercise in musical sadomasochism." (The SPCO's account, without conductor, is wisely spread over two programs; the second half will be heard Nov. 3-5.)

The work has been arranged for innumerable instrumental combinations, from recorder consort and saxophone quartet to full orchestra. The SPCO uses a version for nine players (string quartet plus wind quintet) by flutist Samuel Baron, made half a century ago but still more than serviceable. While Baron's colors (which occasionally suggest Stravinsky) can sound anachronistic, they aid the ear, disentangling contrapuntal strands and injecting needed contrast. Thursday's performance in Stillwater, exemplary in matters of texture, sometimes seemed a little rushed.

Brooklyn-born composer Leon Kirchner studied with Arnold Schoenberg (without adopting Schoenberg's 12-tone technique), became a revered teacher at Harvard and, in 1967, won the Pulitzer Prize. Music, Kirchner said, is "perhaps the last frontier of the irrational, the magical, and the universal"-- and he wrote that way. He died in 2009 at age 90. Thursday's performance of his absorbing "Music for 12" (1985) felt like a memorial.

Despite its egalitarian title, "Music for 12" resembles a chamber-scaled violin concerto, a cousin of Alban Berg's "Kammerkonzert" (to be played by the SPCO late this month). Concertmaster Steven Copes, strongly seconded by conductor Scott Yoo and the ensemble, stressed the work's rhapsodic lyricism and near-tragic intensity, as if to refute critic Kyle Gann's contention that Kirchner and contemporaries, though brilliant, "gave the listener nothing to take home."

The program opened with the G-minor Sinfonia (No. 12) by the Bach-besotted, fugue-fabricating, 14-year-old Felix Mendelssohn, played with grit and a soupçon of mock-seriousness by Yoo and the SPCO strings.