Every face of modernism is the face of Janus. To tell the story of modernism is to tell two stories simultaneously, in dissonant counterpoint—for each story makes itself felt against the antistory that shadows it. For example, one story of modernism concerns aesthetic purism, the strict partitioning of one medium from another. The great narrator of this story is the art historian Clement Greenberg, who writes, “To restore the identity of an art the opacity of its medium must be emphasized.” The puritan Greenberg resists the slightest erasure of the lines that divide one art form from another. He provides a simple criterion for success: a work of art is good to the extent that it displays the substantiality of its medium, without dissembling or fraud. Painting is a thrusting forth of pigment; sculpture is an extancy of metal, an inertia of stone; music is naked sound. The barrier between music and painting (or any other form of art) should be wholly impermeable.

Similarly, the puritan Theodor Adorno considers that Igor Stravinsky had defiled the art of music through what Adorno calls a pseudomorphosis, an attempt to organize a composition in one medium according to alien principles derived from a wrong medium. Adorno accuses Stravinsky of pretending to be a cubist painter: “the spatialization of music is witness to a pseudomorphosis of music to painting, on the innermost level an abdication”; “The trick that defines all of Stravinsky's organizings of form: to let time stand in, as in a circus tableau, and to present time complexes as if spatial—this trick wears off. It loses its power over the consciousness of duration.” Such purism is strongly antiromantic in that it contradicts Wagner's notion of the Gesamtkunstwerk, the total artwork in which the component media fuse into a single gigantic experience.

But every narrative of purism bears witness to a counternarrative that threatens to engulf it. Many composers of the twentieth century were impurists, paying little attention to the warnings of Greenberg; they continued to experiment in erasures of intermedia boundaries, in crossovers from music to painting and other media.