A German-born American artist and educator whose work, both in Europe and in the United States, formed the basis of some of the most influential and far-reaching art education programs of the twentieth century.

His work represents a transition between traditional European art and the new American art. It incorporated European influences from the Constructivists and the Bauhaus movement, and its intensity and smallness of scale were typically European. "Hard-edge" abstract painters drew on his use of patterns and intense colors, while Op artists and conceptual artists further explored his interest in perception.