A work of art may appear unfinished for an array of reasons, including frustration, disinterest, or the artist’s death. The effect is often uncanny. In the case of a painting, subjects drawn only as outlines may seem to have vanished completely from the canvas; inversely, a blank background might make discernible figures float ghostlike in an empty, waiting-to-be-defined space. People might seem monstrous too, with a head drawn but not a face, or a body missing limbs.

These in media res creations have long inspired conjecture about artists’ biographies and psyches—as if they let us see the hand in mid-motion, the artist in mid-thought, and witness the work as only the artist might, before it was deemed fit for public consumption and enjoyment. Many artists work with a rough ...