Part IV Chapters IV-VII
Gulliver explains that the Yahoos are the governing creatures in his country, a fact that the Houyhnhnms struggle to comprehend. Gulliver goes on to describe how the horses are fed and groomed, and how Yahoo servants are employed to look after them. The Houyhnhnms comment that whatever Gulliver claims about the subservient role of horses in his country, it is clear that in reality, the horses are the masters there too. But when Gulliver explains that horses owned by tradesmen are subjected to drudgery, the Houyhnhnms wonder how such a weak animal as a Yahoo can ride upon a Houyhnhnm, since it would be easy for the Houyhnhnm to throw off the Yahoo. Gulliver says that the horses are trained to accept human domination and are beaten if they disobey.
Gulliver tells the Houyhnhnms the story of his life and how he came to their land. The Houyhnhnms find it difficult to understand the vices of the piratical crew members of the ship on which Gulliver arrived.
Gulliver tells his Houyhnhnm master about how society in England is organized, and the war between England and France. The Houyhnhnm asks about the causes of war in Gulliver's society. Gulliver says that sometimes, monarchs feel that they do not have enough land or people to govern; in other cases, wars are pursued to distract the citizens of a country from the evils of the government. Causes of war include differences of opinion, such as the views of Catholics and Protestants about whether the bread and wine used in the holy communion ritual are in reality the flesh and blood of Christ, or symbolic of it. Gulliver says it is considered justifiable to invade a country after it has been weakened by famine or civil unrest. If the people of the conquered country are poor and ignorant, it is thought acceptable to enslave them in order to civilize them. It is also considered acceptable for the leader of a country that helps another country drive out an invader to seize the country for himself.
The Houyhnhnm says that it is lucky that nature has designed Gulliver's type of Yahoo in such a way that they are incapable of doing much harm to each other; their mouths and claws are unfit for fighting. Gulliver smiles at his ignorance and describes the weapons and ammunition that are used in war, including gunpowder, pistols, and cannons. With these, he explains, it is possible to blow up a hundred enemies at once. The Houyhnhnm is disturbed. He does not blame the Yahoos of his own country for their brutal behavior, as that is their nature, but he can see no excuse for a creature "pretending to reason" to act thus.
The Houyhnhnm asks Gulliver to explain what he meant when he said that some of his ship's crew left England as a result of being ruined by law. Law, the Houyhnhnm points out, is meant to sustain life, not destroy it. Gulliver explains the abuses of law in human society. He says a lawyer is a person who is paid to argue that white is black and black is white, and who is more skilled at defending falsehood than justice. It is held among lawyers that any decision that has been made by a court in the past may be made again (the rule of precedent), however unjust it may be. Lawyers use a jargon that no one else can understand. They have encouraged the proliferation and over-complexity of laws. This serves their interest, as it can take years, and correspondingly large legal fees, to settle a case. In the cases of alleged crimes against the state, the judge first finds out the government's view, and passes sentence accordingly.
The Houyhnhnm cannot understand what could motivate lawyers to injure their fellow humans. Gulliver explains the concept of money, which the Houyhnhnms do not have. Money, Gulliver says, enables a Yahoo to buy the finest properties, clothes, and foods, and to have his choice of the most beautiful women. In sum, no Yahoo thinks he has enough of it. He says that for every rich man, there are a thousand poor, and the rich live off the labor of the poor. When Gulliver remarks that the ingredients for rich people's meals have to be transported around the globe, the Houyhnhnm says that England must be very miserable if it cannot be self-sufficient in food. Gulliver says that on the contrary, England produces three times as much food as its inhabitants could consume, but most is exported in return for imports of luxury goods that satisfy the greed and vice of wealthy people. An example is wine, which only serves to create illness and early death.
Gulliver finds it difficult to explain illness to the Houyhnhnm, as it is unknown in their society. The Houyhnhnm says that nature, which works all things to perfection, cannot allow disease to take hold of the body. Gulliver tells him that humans abuse their bodies through harmful habits that disrupt digestion. He says that they acquire syphilis from sexual relations with prostitutes, and that the disease is passed down through generations. Gulliver goes on to detail the frequently destructive medical armory of the doctors of his time, including purgatives and emetics. Some diseases, he says, are imaginary, and the doctors have invented imaginary cures for them. In the case of serious diseases, the doctors predict death, and are proved correct, since while they cannot cure the patient, they can always kill him with toxic medicine. Many doctors are employed as poisoners by people who have wearied of their spouses.
Gulliver describes the qualities needed in a chief minister in the government (the equivalent of the modern prime minister or president of a country). The most important quality is hypocrisy. For example, if the candidate publicly attacks the corruption of the court, the monarch will choose him as chief minister because he knows that such zealots always prove most subservient. The minister preserves himself in power by bribing Members of Parliament. He is able to remain unaccountable to the public because of an Act of Parliament (the Act of Indemnity) that makes him immune to prosecution, and he retires laden with wealth that he has stolen from the nation.
The Houyhnhnm feels sure that Gulliver must have been born into a noble family, because he is cleaner and better-looking than most of his fellow Yahoos. Gulliver corrects him, pointing out that he misunderstands the concept of nobility as it exists in England. There, young noblemen are brought up to be idle and end up marrying an unhealthy and unpleasant woman for her money. So weak and diseased is the nobility that if a nobleman is healthy and robust, he is suspected of being the illegitimate offspring of a groom or coachman. Yet such nobles, in Parliament's House of Lords, have the power to decide the laws of the nation.
Gulliver defends himself against possible accusations that he is representing humankind too honestly to the Houyhnhnms, who are predisposed by their familiarity with Yahoos to a negative opinion of humans. He confesses that living among the Houyhnhnms has inspired in him such love and respect for them that he has no wish to return to human society. Nevertheless, he feels that he has portrayed his own society to the Houyhnhnms in as positive a light as he could.
Gulliver's Houyhnhnm master gives his verdict on Gulliver's fellow man, saying that although he seems to be endowed with a pittance of reason, he only misuses it to corrupt himself. In addition, man appears to lack those natural abilities that Yahoos possess, such as agility. He adds that man's institutions of government and law plainly lack reason and virtue. He considers that the causes of dissent among Yahoos are the same as among humans, particularly greed, since if a group of Yahoos were given enough food for fifty, each would want to keep it all for himself. Moreover, groups of Yahoos fight their neighbor Yahoos with no cause, and if they lack an external enemy, they fight amongst themselves.
Yahoos are fond of shiny stones, which they dig up and hoard in their kennels, while worrying constantly lest their fellows should find their treasure. The Houyhnhnm cannot understand the Yahoos obsession with these useless stones, but he believes that their behavior is similar to avarice among humankind. The Houyhnhnm adds that Yahoos are given to consuming a root that intoxicates them, as wine does humans, which makes them either hug or fight one another. Yahoos, like humans, are subject to diseases of excess.
Each herd of Yahoos has a leader, who is uglier and more mischievous than his fellows, and who is skilled at licking the feet and posterior of his Houyhnhnm master. He is hated by his fellow Yahoos and when he is finally forced from office, the other Yahoos defecate on him. Gulliver's Houyhnhnm master leaves Gulliver to decide whether this relates to human court favorites and government ministers.
The Houyhnhnm mentions the Yahoo habit of sitting and howling in a corner for no reason. The only remedy is to set the Yahoo to hard work. Gulliver recognizes this as a disease of wealthy and idle Englishmen, which is known as spleen. The Houyhnhnm describes the odd behavior of female Yahoos, which Gulliver recognizes as (in his view) the human female tendencies of flirting, blaming and judging others, and malicious gossip.
This section of the book marks a change in Gulliver's attitude to his fellow men. While previously, he has assumed a certain superiority to the people around him (who, unnoticed by him, are caricatures of people from his own society), under the influence of the rational Houyhnhnms, he now feels disgust at his own kind. Humans are portrayed as being worse than Yahoos, since Yahoos are not rational and therefore have no choice but to act in a bestial manner, but humans pretend to rationality and therefore have a choice as to how to act. That humans should choose greed, avarice, and hypocrisy over the selfless rationality
exemplified by the Houyhnhnms is shown as morally reprehensible as well as absurd.
Yahoo qualities that are described in order to satirize human vices include greed, avarice, and an aggression that leads Yahoos to fight their own kind. The flirtatious and malicious behavior of female Yahoos is also calculated to reflect the behavior of female humans. The despicable behavior of leader Yahoos reflects the hypocrisy of government officials in human society.