In "A Modest Proposal," narrative tone must be separated from authorial tone: the narrator, or the "Proposer," is distinct from Swift himself. The Proposer's tone is earnest, persuasive, and rational; all of these tonal aspects add to the horror of the proposal. To earnestly suggest child cannibalism as a valid solution without any tonal reservations is shocking indeed. In the following passage, for instance, the Proposer states the importance of his proposal with no sarcasm whatsoever:
I think the advantages by the proposal which I have made, are obvious and many as well as of the highest importance.
What readers might perceive as a sarcastic tone in this passage is actually irony, stemming from the inherent contradiction in calling child cannibalism a "proposal" with "advantages." Herein lies the distance between narratorial and authorial tone: the Proposer, in this passage, employs an entirely sincere tone, believing every word of what he says. Swift, on the other hand, utilizes a satirical tone to critique the Proposer and other essayists like him. To put it another way, the Proposer is quite sincere in his suggestion, but when read with the knowledge that the essay is satirical, the tone becomes decidedly tongue-in-cheek.
This distance between the narrator's tone and Swift's tone is crucial to the text's satirical purpose. Viewed through the eyes of satire, the Proposer's supposed "rationality" becomes farcical: an inane facade behind which violence is considered a legitimate solution to further human suffering.