In Book Four, Gulliver travels to the land of the Houyhnhnms, morally upright, reasonable, talking horses. The Houyhnhnms rule over the savage, human-like Yahoos, which resemble Gulliver and other Europeans. Through Gulliver's interactions with the Houyhnhnms, Swift satirizes the European belief that humans are morally superior beings, who are ruled by dignified, honorable governments. Gulliver has several conversations with his Houyhnhnm master in which he attempts to explain European culture. However, Gulliver's master cannot comprehend many of the ills that plague European society. Gulliver's Houyhnhnm master has difficulty understanding Gulliver's description of the numerous vices inherent in mankind because there are no words for those wicked exploits in their country.
In chapter 5 of Book Four, Gulliver has another conversation with his Houyhnhnm master, in which he attempts to describe the nature of European war. Gulliver begins by describing the ridiculous, petty reasons European aristocrats go to war, then proudly describes the weapons used to slaughter thousands of enemies. Gulliver's description appalls the Houyhnhnm, and Gulliver mentions,
"But as my discourse had increased his abhorrence of the whole species, so he found it gave him a disturbance in his mind to which he was wholly a stranger before" (Swift, 314).
Swift once again satirizes the European belief that humans are morally superior beings through the reaction of Gulliver's Houyhnhnm master, who finds it utterly detestable that a Yahoo claiming to be reasonable could inflict such destruction upon other Yahoos.