"The Knife" is one of nineteen pieces of writing in Richard Selzer's book Mortal Lessons: Notes on the Art of Surgery. Its genre is difficult to define, as it sometimes appears to be an essay and at other times a short story. The point of view is similarly flexible, starting with the surgeon, moving on to the author's own memories of childhood, and moving on to the patient, only to return to the surgeon's perspective at the end. Even more unusually, the narrative employs not only both the first and third person, but also the second, putting the reader in the position of the surgeon.

The piece describes a surgical operation, digressively and from all these different points of view. It begins and ends with the knife of the title, first with a description of how to hold it, finally with the knife resting, waiting for the next operation. The disorienting change in perspectives emphasizes what a strange thing it is for one person to slice another open. This is the central theme of the piece: the unnatural nature of surgery. By the end of the piece, the operation is the stuff of nightmares:

The breasts of women are cut off, arms and legs sliced to the bone to make ready for the saw, eyes freed from sockets, intestines lopped. The hand of the surgeon rebels. Tension boils through his pores, like sweat. The flesh of the patient retaliates with hemorrhage, and the blood chases the knife wherever it is withdrawn.

This is a stark contrast with the earlier descriptions of surgery as a vocation of mercy, purification, and even love. As Selzer remarks, however, "One can count on absurdity."