Michael Craig-Martin’s work becomes more concrete with the passing years. His black-and-red-tape line drawings on gallery walls gave way to outlines defined in black-painted steel, and some of his most recent works (12 of which were shown here, together with some pieces from 1986) are almost freestanding objects. The new works, all from 1987, incorporate elements of the steel outlines within and around aluminum-framed slices of “reality.” All of his work sets out to probe notions of what art is and how it functions. And in his use of found objects and his references to the essence of others,