Part IV Chapters VIII-XII
With a servant Houyhnhnm as his guard Gulliver goes to investigate the Yahoos to see whether they are similar to humans. He finds them smelly, unteachable, and cruel. A female Yahoo sees Gulliver naked and leaps upon him in a fit of lust. This convinces Gulliver that he must be a Yahoo, since the females desire him as they would one of their own species.
Gulliver describes the principles of the Houyhnhnms. They are naturally virtuous, cultivate reason, and have no concept of anything evil. They act with friendship and kindness towards their fellows, and the only factor that makes them give a distinction to any particular Houyhnhnm is if he is unusually virtuous.
In order to avoid overpopulation, Houyhnhnms only mate to produce one offspring of each sex. Genetically inferior Houyhnhnms, who are destined to be servants, are allowed to breed three offspring of each sex to serve in noble families. They are careful to make matches that preserve the strength and good health of the race as a whole. Courtship and romantic love have no place in the process, but the couple displays the same kindness and respect to each other as to the rest of their race.
Female Houyhnhnms receive the same education as the males, and they are brought up to behave virtuously. All Houyhnhnms receive physical education to make them strong and healthy.
Every four years, the Houyhnhnms hold a council to identify any food shortages in the districts, which are immediately rectified.
A council of the Houyhnhnms is held at which it is discussed whether to exterminate the Yahoos from the face of the earth. The case for extermination is that the Yahoos destroy the Houyhnhnms's crops and steal their milk. They are only useful for carriage, a role that could be performed by asses, which are less troublesome.
Gulliver's Houyhnhnm master suggests that they could castrate the Yahoos at an early age, as Gulliver's people castrate their horses, to make them tame and to put an end to their race without taking life. Meanwhile the Houyhnhnms could begin to domesticate asses.
The Houyhnhnms have no written language. However, they are skilled in herbal medicine, practical astronomy, and poetry. Their poetry is geared towards inspiring readers to greater virtue or physical excellence.
Houyhnhnms (in the absence of accidents) die of old age. Their friends and relatives express neither joy nor grief at their departure, as they believe that death is simply a return to the "first mother."
Houyhnhnms have no word to express anything evil. They describe mistakes or unpleasant events by adding the word "Yahoo" to the word or phrase.
Gulliver is happy living a simple life among the Houyhnhnms. He dresses in animal skins and is relieved to have no contact with the follies and vices of human society. Gulliver feels privileged to be allowed to listen to the conversation of his Houyhnhnm master, which contains much wisdom. He comes to love and respect the Houyhnhnm race. He thinks of his family, friends, and countrymen as Yahoos. As he feels that he is himself a Yahoo, he is filled with self-disgust.
One day, Gulliver's Houyhnhnm master summons him and tells him that at the council, the other Houyhnhnms had voiced their disapproval of his keeping a Yahoo (Gulliver) as a member of his family. This was contrary to reason and nature, so the Houyhnhnms had decided that Gulliver should either be employed like the rest of the Yahoos or sent back from whence he came. They fear that Gulliver's rudiments of reason will make him a dangerous leader who could raise the Yahoos in rebellion against the Houyhnhnms.
Gulliver is grief-stricken at the prospect of leaving the Houyhnhnms and going back to England to live among Yahoos. He takes his leave of his Houyhnhnm master and sets sail in a boat he has constructed.
At sea in his boat, Gulliver hopes to find an uninhabited island where he can live, rather than return to England. He finds what he believes is such an island, goes ashore, and sets up home. On the fourth day, he sees some natives, who fire an arrow at him and wound him in the knee. Gulliver escapes in his boat and goes ashore next to a creek on the same island. A ship draws near and sends some men ashore to get fresh water. They discover Gulliver and address him in Portuguese. Gulliver baffles them by describing himself as a Yahoo and by his manner of speech, which resembles the neighing of a horse. They tie Gulliver up and take him on board their ship. The captain, Don Pedro de Mendez, treats Gulliver with great courtesy.
Gulliver tries to escape by leaping overboard rather than live among Yahoos, but is recaptured. Don Pedro gently explains that he only wants to help Gulliver. Gulliver tells the story of his life among the Houyhnhnms. Eventually, Don Pedro begins to believe him. Gulliver passes the journey trying to avoid contact with the crew because he has developed an aversion to humankind. He refuses Don Pedro's offer to lend him his best clothes, but Gulliver cannot bear to wear anything that has clothed a Yahoo. The ship arrives at Lisbon and Don Pedro accommodates Gulliver as his guest in his house. At first, Gulliver is too terrified of humans to venture outside. After some time, he agrees to go out in the company of Don Pedro, but insists on blocking his nose with herbs or tobacco to lessen the unpleasant smell of humankind.
Don Pedro persuades a reluctant Gulliver to return England. He does so, but feels only disgust at his wife and family. For a year, he cannot tolerate their presence. He buys two horses and converses with them in their stable for four hours each day.
Gulliver concludes his story by asserting the truth of his account and condemns those travel writers who lie and embellish. His motive, he says, is to serve the public good by drawing attention to the virtues of the Houyhnhnms and thereby to the vices of humankind. He adds that among the Yahoo nations, the least corrupted are the Brobdingnagians.
Gulliver is pleased that his work can meet with no criticism or blame, for all he has done is relate the plain facts, without his own opinions.
Gulliver has been advised that he must give an account of his discoveries to the Secretary of State, as any lands that are discovered by an English subject belong to the Crown (monarch). He has not done so, as it is not worth sending an army to conquer Lilliput, and it would not be safe or wise to invade Brobdingnag, Laputa, or the land of the Houyhnhnms. In addition, he does not wish to bring the violence and destruction of an invasion upon these nations. He quickly adds that he does not accuse the English of such behavior, as they are well known for their wisdom and justice in establishing colonies. But the nations he has visited do not wish to be conquered and enslaved, nor do they possess any valuable minerals or crops that the English government might be interested in, so he sees no point in making such a report.
Recently, Gulliver has begun to allow his wife to sit at dinner with him, but he has to block his nose with herbs to disguise the smell. Perhaps one day, he may be able to tolerate the company of a fellow Yahoo. He finds himself particularly provoked by the combination of vices and pride in humankind. Houyhnhnms, though they are virtuous, utterly lack pride, and he hopes that any human who possesses this absurd vice will refrain from coming near him.
The Houyhnhnm society is similar to that of the ancient Spartans, which, by the 5th century B.C., was the most powerful nation among all of Greece's city-states. Both societies had no written literature, but passed down laws and customs orally. Both societies emphasized morality and the importance of doing that which is right. Both societies regulated childbirth in the interests of society as a whole: the Spartans left weak or malformed babies to die to keep the race genetically strong, and the Houyhnhnms choose their mates with a similar aim. Both societies had the state educate children; both emphasized physical fitness, though with the Spartans, this last custom had a military purpose not shared with the Houyhnhnms.
Though Gulliver describes the rational Houyhnhnm society with complete approval, it has attracted negative views from some modern critics and readers. In this society, the individual is subjugated to the interests of society as a whole. Houyhnhnms choose their mates for the good of the race, not out of personal preference, and feel no special emotions for their mates or children. Families with a surplus of children pass them, with complete equanimity, to another family that has a vacancy. They do not grieve for dead loved ones.
This is a society, some critics point out, that is completely without the conflicts, hatreds, passions, strong preferences, and loves that define the texture of human society and that distinguish humans so markedly from one another. It is no coincidence, these critics add, that the Houyhnhnms are the only race visited by Gulliver who do not possess individual names. One Houyhnhnm is completely indistinguishable from another.
No man can be entirely a Houyhnhnm (rational) or entirely a Yahoo (bestial). But he can be a Don Pedro. Don Pedro exemplifies such human virtues as kindness, charity, compassion, and generosity. Gulliver, who has been warped out of his humanness through excessive exposure to the Houyhnhnms, is blind to Don Pedro's goodness simply because the man does not look like a horse.
The reader, who does not share Gulliver's fit of insanity, can appreciate it.
There is irony in Gulliver's final condemnation of his fellow humans because of their pride. This is because Gulliver himself has become insufferably proud, feeling superior to everyone else because of his over-identification with the Houyhnhnms. Gulliver is not, and can never be, a Houyhnhnm, as is clear from their expulsion of him from their realm on the grounds that he is not one of them.