"The Author sets out as Captain of a Ship. His Men conspire against him, confine him a long Time to his Cabin, and set him on Shoar in an unknown Land. He travels up into the Country. The Yahoos, a strange Sort of Animal, described. The Author meets two Houyhnhnms."
After five months at home, Gulliver leaves his children and pregnant wife yet again to go on his fourth voyage, this time as captain. Not long into the trip, his crew mutinies, locking him into his cabin for a great deal of time and threatening to murder him. Eventually the crew, who plan to become pirates, drop Gulliver off on an unknown island.
Gulliver walks inland until he comes across a field of strange creatures. After observing them for some time he comments, "Upon the whole, I never beheld in all my Travels so disagreeable an Animal, nor one against which I naturally conceived so strong an Antipathy." Soon Gulliver comes to realize that these are actually naked human beings behaving like cattle. Gulliver comes face to face with one of them. He hits it with the side of his blade when it comes at him violently. The animal-like human (which Gulliver later learns is called a Yahoo) cries out, causing the rest of the forty Yahoos to surround Gulliver.
Gulliver fears the worst until the Yahoos suddenly flee because of a grey horse coming toward them. The horse takes an interest in Gulliver and circles him until another horse comes along. Gulliver observes that their whinnies to each other sound almost like a language. Gulliver hears the word Yahoo several times and repeats it to the great surprise of both horses. The horses then teach Gulliver the word Houyhnhnm, which Gulliver later learns is their word for themselves-for horse. Afterward, the grey horse signals to Gulliver that he should walk in front of him, which he does.
"The Author conducted by a Houyhnhnm to his House. The House described. The Author's reception. The Food of the Houyhnhnms. The Author in Distress for want of Meat. Is at last relieved. His Manner of feeding in this Country."
Gulliver and the grey horse arrive at a home where Gulliver expects to meet the horse's human masters. The two move through every room of the house and meet several other horses before Gulliver realizes that the grey horse is the master of the house.
After some discussion between the horse and his wife about whether or not Gulliver is in fact a Yahoo, he is brought out to the stable where the Yahoos are kept and is made to stand next to one of them. Aside from the extra hair, longer nails, and nakedness of the Yahoo, they are the same.
Gulliver makes a kind of bread out of the horses' oats for his dinner and is given a small room near the house with some hay to sleep in.
"The Author studies to learn the Language. The Houyhnhnm his master assists in teaching him. The Language described. Several Houyhnhnms of Quality come out of Curiosity to see the Author. He gives his Master a short Account of his Voyage."
After about three months of living among the Houyhnhnms, Gulliver has learned their language quite well and can answer most of their questions. He tells them about the mutiny that landed him on their shores, but they have a very difficult time understanding, because they have no concept of what a lie is. They tell Gulliver that "The Word Houyhnhnm, in their Tongue, signifies a Horse, and its Etymology, the Perfection of Nature."
The horses believe that Gulliver is a Yahoo-but a more rational and civilized Yahoo. Gulliver, wanting to separate himself from the Yahoos as much as possible, asks not to be called a Yahoo anymore.
"The Houyhnhnms' Notion of Truth and Falsehood. The Author's Discourse disapproved by his Master. The Author gives a more particular Account of himself, and the Accidents of his Voyage."
Gulliver continues explaining the concept of lying to his master. He also explains the relationship of horses and humans back in England. The horses cannot believe that humans would be able to control creatures that are so much stronger than they are, but Gulliver explains that horses are tamed beginning at a very young age.
"The Author at his Master's Commands informs him of the State of England. The Causes of War among the Princes of Europe. The Author begins to explain the English Constitution."
Over the next two years, Gulliver explains much about the English government and political systems. Gulliver tries to explain war and the reasons why humans kill each other. His master says that Yahoos in England are worse than Yahoos because they use their reason to gain power but use it badly.
"A Continuation of the State of England. The Character of a first Minister."
Gulliver continues telling his master about the vices of the English people. He paints a particularly disturbing picture of lawyers and doctors, saying that lawyers are the stupidest among the Yahoos and doctors are corrupt and seldom cure their patients.
In the country of the Houyhnhnms, Gulliver meets the species that is the most skeptical of him-and for good reason. Gulliver must do everything he can to separate himself from the Yahoos, a very different situation from his distinct positions in Lilliput and Brobdingnag. In order to accomplish this, Gulliver does small things daily like using his best manners, eating with a knife and fork, keeping his clothes on, and being as clean as possible. He shows that he can use language, can reason well, and can be prudent and mannerly.
It is interesting to note that from the very beginning of his time in the country of the Houyhnhnms, Gulliver strives to separate himself from his own species. Is this what Swift has been trying to do his entire life? It often is difficult to strive for individual human greatness among a mass of people who hardly try and have hardly any notion of what greatness would be. In Brobdingnag, when Gulliver explained the English people and their way of life to the king, the king decided they were lowly creatures and Gulliver became offended, trying to defend his people. Something is different now in the country of the Houyhnhnms. When the grey mare tells Gulliver that he thinks his people are worse than the Yahoos, Gulliver is quick to agree.
What is different here? Only Gulliver's experiences since Brobdingnag and his contact with the Yahoos. Through the Yahoos, Gulliver has come to see some awful aspects of human nature, and Swift has shown his readers what they would be (and often are) without the intelligence and graces of which they are capable. Gulliver seems willing to turn his back on the English people in favor of those he deems better than the English. Now that he has been exposed to many alternatives, he can think carefully about who to admire and what political systems to favor, and the English certainly come up short in relation to the Houyhnhnms.
Also interesting in these chapters is Gulliver's plain admonishment of lawyers and doctors. Gulliver's negative commentary about lawyers is in many ways not surprising except for its level of ferocity. Lawyers seem no better than politicians, going to court over the petty human squabbles that Gulliver satirized as early as Part I. Gulliver's description of doctors as shallow and greedy people who would kill a patient as soon as cure him is surprising to contemporary readers, especially because Gulliver has spent so many years working as a surgeon. One should remember that eighteenth-century medicine was still rather poor.