As its name implies, the Henry Moore Institute is dedicated to the display and study of sculpture. All the more intriguing, therefore, to find it staging an exhibition of work by Imi Knoebel. Despite the fact that it extends so often into three dimensions, Knoebel’s work—like that of Blinky Palermo, his fellow student at the Kunstakademie Düsseldorf—is implacably that of a painter. Under the title “Primary Structures: 1966/2006,” this thoughtful, concentrated show places works from the beginning of Knoebel’s career alongside a group of very recent pieces, including two panel paintings and a