“Structure” is the theme as artists Matthew Ritchie, Fred Wilson, Richard Tuttle and Roni Horn are profiled. “Modern art is a gift,” says Ritchie, whose room-sized installations are rooted in line drawing. Wilson's installations are formed by “putting things together,” not making them. Tuttle, a veteran painter-sculptor, likes to explore “the part which I can't see,” as does Horn, a sculptor-photographer. “The unknown,” she says, “is where I want to be.” Introduced by Sam Waterston.