THE PIANIST CRAIG TABORN described how, observing the venerable Art Ensemble of Chicago as a young man, the group’s five members would warm up, practice, and start playing backstage before the show, so that “by the time the concert began the music had already been happening . . . they were simply bringing it out with them.” Likewise, Raoul De Keyser’s art gives the distinct impression of having been carried from somewhere upriver, of having coalesced long before taking form on canvas—and of lingering in the air even after you’ve stopped looking at it. When De Keyser began to gain a reputation