Gulliver's Travels: Travels into Several Remote Nations of the World.

Gulliver begins to realize that, due to his experiences with the Houyhnhnms, he has developed contempt for his fellow humans. He is struck by the contrast between the Houyhnhnms’ natural virtue and the numerous, omnipresent vices of humanity. After a year, his appreciation for the quadrupeds grows to such an extent that he resolves to never return to humankind again.

The master Houyhnhnm points out that while Gulliver shows faculty of reason, in some ways, his European demeanor shows him to be physically softer and more ill-equipped than the Yahoos. The master Houyhnhnm concludes that although there remain differences between Gulliver and the Yahoos, effectively they are the same. Therefore, in his estimation, humans are Yahoos. The master Houyhnhnm also discusses ways in which rudimentary envy manifests itself in the Yahoos when they see shiny rocks and covet them. Often, this envy leads to violent aggression. He contends that of all animals, Yahoos love dirt more than any other, an opinion that Gulliver points out would likely change if the master Houyhnhnm had seen pigs. Finally, the master Houyhnhnm asserts that female Yahoos use sex as a means of seducing male Yahoos, a point which enables Gulliver to demonstrate his derogatory view on women by claiming that female humans effectively do the same thing.