Image: The whistle, posted at Flickr by Amanjeev under a Creative Commons License. Image cropped.

Willa Cather says, “The heart of another is a dark forest, always, no matter how close it has been to one’s own.” What might strike some as an inscrutable terror – how dare we be denied ultimate knowledge of our most intimate? – is an occasion for joy in Mary Oliver’s “The Whistler”.

The scene is tender, domestic, everyday: the speaker is reading in the upstairs bedroom, and their companion, who...