Back in the 1950s and 60s, when the American artist Alex Katz was a young man, Abstract Expressionism was dominant. Katz quietly rejected it, looking for something new. “The one thing I don’t want to do is things already done,” he once said. “As for particular subject matter, I don’t like narratives, basically.” He created and then destroyed thousands of works, until he landed on his signature technique. Katz is a figurative painter who conjures portraits and landscapes—large ones—from flat planes of color that puzzle perception. The images look simple, but they are not. There’s something existential at play. In Katz’s first retrospective in Spain, 40 large-format paintings are on view. —E.C.