¶ … fantastical voyage in Gulliver's Travels, Gulliver encounters a race of highly intelligent horses whose extreme rationality seduces the protagonist. Gulliver's increasing hatred for humanity becomes a dark vehicle for Swift's through satire of human nature. The Houyhnhnms embody Enlightenment ideals, as they are led by reason over emotion and essentially devoid of passion. However, the horses nevertheless exhibit prejudice in their treatment of the Yahoos and Gulliver. The Houyhnhnms represent qualities that human beings often blindly strive toward, and Swift shows that pure reason is not necessarily superior to the nuances of human emotion. Gulliver perceives the Yahoos through the Houyhnhnms' eyes, as horrible brutes. Because of their resemblance to human beings in physical and psychological makeup, Gulliver begins to despise humanity. Swift thus presents a paradox: Gulliver's perception of human beings is in many ways correct. However, his sweeping generalizations of human nature results in unproductive behavior. For instance, he refuses to be rescued by the kind Portuguese sailor. After being forced to return to England, he isolates himself from his family and retreats into his own world. Through the examples of the Houyhnhnms, the Yahoos, and of Gulliver, Swift uses Part Four of the book to elucidate and satirize the many shortcomings of mankind. The voyage to the country of the Houyhnhnms begins with a mutiny. From the outset, this section of Gulliver's Travels denotes the disloyalty inherent in human nature. His crew resolves to become pirates, moreover, a profession that connotes immorality because it usually entails robbery and assault. The mutinous crew confines Gulliver to a cabin and sends him off to sea, an act of abject cruelty. Therefore, human beings are shown to be innately nefarious, selfish, and greedy. When Gulliver arrives on the Houyhnhnms' territory, he is besieged by Yahoos. At first, the animals do not seem human at all to Gulliver. They are described as being ape-like: they have hairy bodies and climb trees. Gulliver is totally repulsed by them: "Upon the whole, I never beheld, in all my travels, so disagreeable an animal, or one against which I naturally conceived so strong an antipathy," (Chapter One). Unaware that these are the same creatures that left the human footprints he saw, Gulliver attacks one of the Yahoos. As they all flee, Gulliver judges them to be cowardly.

In Chapter Two, Gulliver discovers that Yahoo physiology much resembles his own: "My horror and astonishment are not to be described, when I observed in this abominable animal, a perfect human figure." Only his use of clothing distinguishes his body from that of the Yahoos, and the horses judge Gulliver to be a different type of creature altogether. Like he does in many other parts of Gulliver's Travels, Swift pokes fun of the anatomy, physiology, bodily functions, fluids, and excretions of humans. Our very physical nature is satirized, in addition to our moral and psychological makeup. Furthermore, although the Houyhnhnms are horses and their physiology completely differs from that of human beings, Gulliver relates with them more than he does with the Yahoos, whose anatomy closely resembles our own. Here begins Gulliver's dissociation with his humanity and his denial of true human nature. Swift implies that human nature is a composite of the Yahoo and the Houyhnhnm: we are part brute animal and part enlightened and rational creature. Gulliver favors the Houyhnhnm nature because the Yahoos are physically and morally repugnant. But in doing so, he suppresses his true nature in an unhealthy manner. Swift demonstrates...

By relating more closely to horses than to humans -- for when he returns to England he continues to speak in horse tongue -- Gulliver not only demonstrates the pitfalls of denial but also draws our attention to the immutability of human nature. To deny this nature is to self-inflict psychological pain and isolation. Swift shows that human nature is immutable by emphasizing physicality throughout Gulliver's Travels. Moreover, Gulliver's friendship with the Houyhnhnms is illusory; they never truly accept him as one of their own. They can't because his physical nature bars them from being able to see him as a complete equal. At best the Houyhnhnms see Gulliver, and through him all humans, as being highly advanced Yahoos.
Swift comments on the fundamental immutability of human nature while satirizing its many glaring faults. Despite what Gulliver may think, these faults are exemplified through both the Yahoos and the Houyhnhnms. Because the Houyhnhnms are horses, they appear immune from the type of criticism reserved for the human race. They seem to Gulliver at first to be some type of magical manifestation of human nature; when it becomes clear that the horses are unique beings, Gulliver is certain that humans are inferior to them based on his judgment of the Yahoos. Especially because they so closely resemble Yahoos, humans are shown as being nothing more than animals with greater intellectual capacity. Swift exaggerates human flaws through his depiction of the Yahoos: they are physically ugly, morally bereft, and intellectually stunted. They do not learn new things and although they are physically strong, the Yahoos act in a cowardly fashion. Interestingly, at this early stage in his stay with the Houyhnhnms, Gulliver states, "there were few greater lovers of mankind at that time than myself," (Chapter Two). After his previous travels, Gulliver retains a great degree of respect for the human race, even if he finds it odd at times. However, this respect for mankind quickly wanes as Gulliver perceives the many unsavory similarities between his species and that of the Yahoos. In Chapter VIII, Gulliver performs an extensive examination of Yahoo culture. He concludes that these "odious animals" are "strong and hardy, but of a cowardly spirit, and, by consequence, insolent, abject, and cruel." Moreover, because the females are attracted to him, Gulliver concludes, "I was a real Yahoo in every limb and feature." Gulliver proceeds to laud Houyhnhnm culture as representing the pinnacle of civilization and what humanity should be striving toward. As a whole, Part IV of Gulliver's Travels transforms Gulliver's perception of humanity altogether because of his exposure to the Houyhnhnms and the Yahoos.

Unbeknownst to Gulliver, however, each of these species symbolizes a facet of human being; Gulliver simply chooses the side that embodies reason. Swift shows that Houyhnhnm culture is imperfect as well: although they are fundamentally benevolent beings, "courtship, love, presents, jointures, settlements have no place in their thoughts, or terms whereby to express them in their language," (Chapter Eight). However, much Gulliver admires the Houyhnhnms, Swift shows that they can be cold, unfeeling, and prejudiced. Swift also provides some social commentary on the perception of the "other" by a dominant social group. The Houyhnhnms don't quite know what to make of Gulliver, as he does not appear to be fully Yahoo in nature and of course does not resemble a horse. The Houyhnhnms accept Gulliver tentatively; he is still considered a stranger and is treated as an "other" even though he is noticed as being far more intelligent than the Yahoos who he resembles. Finally, in Chapter Ten the Houyhnhnms banish Gulliver; they were unable to see him as an individual and could not fully grasp human culture. In spite of his being exiled and unfairly judged, Gulliver continues to idealize the Houyhnhnms to the point of only speaking in horse-tongue when he returns to England. Even though Gulliver did not totally fit in with the Houyhnhnms' culture, he loves and respects the horses mainly for their extreme rationality, which is a distortion of the human scientific impulse. He begins to make of himself a stranger to the human race; when he eventually returns he essentially denies his own humanity. Likewise, the Yahoos are an exaggerated example of…