Western fine art has had appropriation in its bones since the High Renaissance. That’s when sacred works, originally meant to please or appease a deity, were first held up for detached aesthetic contemplation—they were appropriated, that is, into a brand-new sphere called “art.” This unacknowledged “borrowing” got a new, more public impetus about a century ago, when the modernists invited us to see every kind of artifact, from airplane propellers to tribal totems, through an artistic lens.

In its appetite for objects from beyond the art world, modernism had a special focus on what’s now known as “outsider art”—work by children, the self-taught, or the mentally ill that looked a lot like what was coming from the studios of the avant-garde. Early curators at the Museum of Modern Art, and around the country, went on an eager hunt for such “untrained masters.” (MoMA quickly lost interest in outsiders. In 1942, founding director Alfred Barr displayed a shoeshine stand by one of them; the millionaires on his board dismissed him for it.)