Hilma af Klint and Piet Mondrian both started their careers as landscape painters. But in 1906, af Klint brought her work inside, into the basement of her house in Sweden, where she drew strange spiritual symbols that took patterns and geometries from nature. Similarly, when Mondrian moved to Paris in 1911, he left his landscapes behind and began searching for a “universal beauty,” a quest that took him through Cubism to pure abstraction. The Tate Modern’s blockbuster exhibition presents these two pioneers together—a first. —E.C.