The credo formulated by Sol LeWitt in the late 1960s, the idea becomes a machine that makes the art, was meant to assert—more radically than his minimalist works throughout the 1960s had—the primacy of the conceptual dimension of an artwork over qualities specific to its material or medium. The principles presented in his Paragraphs on Conceptual Art (1967) and Sentences on Conceptual Art (1969) had a decisive influence on the conceptual art agenda in the United States. LeWitt’s transition from minimal to conceptual art comes to light both in the Wall Drawings he began to create in 1968 and in sculptural works such as Incomplete Open Cubes. LeWitt’s so-called working drawings serve, on the one hand, to establish the rules by which the Incomplete Open Cubes are combined, and, on the other hand, as instructions for implementing the work. The sequencing that here underpins the systematic arrangement of the individual forms has been identified by Pamela M. Lee as phase shifting—a method borrowed from minimal music, the art historian deduces, in the wake of LeWitt’s preoccupation with the compositions of Steve Reich. The method entails the drifting apart of audio loops played back simultaneously, which LeWitt sought to translate as a contingent aggregation of visual fragments. As Lee writes, LeWitt wished to render aesthetic perception visible as an incomplete process composed of intuitively interlinked sense perceptions. In order to run through all the possible variations of the three central parameters of the work—cube, seriality, and incompleteness—LeWitt used letters and numbers in his working drawings. They were aids to logical systematization—that is, to ensuring that all possible variations were considered and identical repetitions avoided. In other words, LeWitt appropriated the method of phase shifting, in the sense of a (self-)difference of the form—in this case, that of the incomplete open cube.
 Pamela M. Lee, “Phase Piece,” in Sol LeWitt: Incomplete Open Cubes, exh. cat., ed. Nicholas Baume (Cambridge, Mass.: MIT Press, 2001), 49–58.