Sol LeWitt, whose deceptively simple geometric sculptures and drawings and ecstatically colored and jazzy wall paintings established him as a lodestar of modern American art, died yesterday in New York. He was 78 and lived mostly in Chester, Conn.

The cause was complications from cancer, said Susanna Singer, a longtime associate.

Mr. LeWitt helped establish Conceptualism and Minimalism as dominant movements of the postwar era. A patron and friend of colleagues young and old, he was the opposite of the artist as celebrity. He tried to suppress all interest in him as opposed to his work; he turned down awards and was camera-shy and reluctant to grant interviews. He particularly disliked the prospect of having his photograph in the newspaper.

Typically, a 1980 work called “Autobiography” consisted of more than 1,000 photographs he took of every nook and cranny of his Manhattan loft, down to the plumbing fixtures, wall sockets and empty marmalade jars, and documented everything that had happened to him in the course of taking the pictures. But he appeared in only one photograph, which was so small and out of focus that it is nearly impossible to make him out. His work — sculptures of white cubes, or drawings of geometric patterns, or splashes of paint like Rorschach patterns — tested a viewer’s psychological and visual flexibility. See a line. See that it can be straight, thin, broken, curved, soft, angled or thick. Enjoy the differences. The test was not hard to pass if your eyes and mind were open, which was the message of Mr. LeWitt’s art.

He reduced art to a few of the most basic shapes (quadrilaterals, spheres, triangles), colors (red, yellow, blue, black) and types of lines, and organized them by guidelines he felt in the end free to bend. Much of what he devised came down to specific ideas or instructions: a thought you were meant to contemplate, or plans for drawings or actions that could be carried out by you, or not.