Throughout Gulliver's Travels, the different peoples that Gulliver visits symbolically represent different aspects of humanity.
Gulliver represents an everyman, a middle-class Englishman who is fundamentally decent and well-intentioned. In the course of his travels, he becomes less tolerant and more judgmental of the nations he visits and of his fellow human beings.
The Lilliputians, a tiny race of people, represent much of what is petty and small-minded about the English and humankind in general. They are physically and morally smaller than Gulliver. They are pompous, self-important, self-serving, hypocritical, and surprisingly dangerous and cruel in spite of their small size. The fact that several persons and events from English political life are widely believed to be referred to in this section reinforces the notion that Lilliput partly represents England. Swift makes the Lilliputians tiny in order to puncture the self-importance of the English nation and of humankind. For example, though the Lilliputians are proud of their military, Swift has them parade their army beneath Gulliver's legs in full view of his nether regions. This is a ridiculous and undignified image which undermines the Lilliputian pretensions to grandeur.
The Brobingnagians, the race of giants, are physically and morally bigger than Gulliver. While vice does exist in their country, unlike humans, they have not built vice into their government and institutions. Therefore, they represent much of what is good in humankind. The Brobdingnagian king is shocked at Gulliver's account of English politics and society, and refuses his offer of gunpowder as he cannot conceive of any good coming from it. However, the great size of the Brobingnagians means that Gulliver can never feel safe or equal in their society; while they treat him kindly, they also treat him as a plaything or an exhibit. This aspect of Brobingnagian society both represents the importance of physical size and power and draws attention to the relative and unreliable nature of power: while Gulliver was large and potentially powerful in Lilliput, he is powerless in Brobdingnag. Swift means this as a warning to nations, such as the English of his time, that the arrival of a larger or more powerful force can easily put an end to their dominance on the world stage.
The Laputans represent the dangers and limitations of abstract and theoretical knowledge. This field was growing in dominance in Swift's time, under the influence of what became known as the Enlightenment. When Swift wrote this section of the novel, most of the experiments and theories espoused at Laputa's Lagado Academy, including extracting sunbeams from cucumbers, had actually been carried out or proposed by the scientists of the Royal Society of London for the Improving of Natural Knowledge, a society founded in 1660 which as of 2006 continues under the shortened name, the Royal Society.
The Laputan people's addiction to abstract knowledge make them oblivious to each other and to all human concerns. This makes them require the ludicrous custom of "flappers" to alert them to listen or speak, and means that their unimpressed wives have adulterous relationships under their noses. The fact that the King of Laputa inhabits an island that floats above his domain is symbolic of his ungrounded thinking and his separation from his people and their practical concerns.
The Houyhnhnms represent reason and virtue. They operate their society according to these principles and as a result, have no crime, shortages, disease, or other problems. They subjugate their own individual lives and concerns to the good of their society as a whole. So deep-rooted is this tendency that they have no distinguishing characteristics or names, and they do not seem to possess an emotional life beyond treating everyone with respect and kindness. While they represent the rational faculty that man possesses, they do not seem fully human and, indeed, expel Gulliver from their society because they see him as a Yahoo. This suggests that Swift does not intend their nation to be seen as a complete and self-contained model for an ideal human society. Rather, their way of life exemplifies much that is admirable and that may be emulated by human beings.
The humanoid Yahoos represent all that is bestial, low, and despicable in human behavior. Gulliver is ashamed to recognize the similarities between them and human beings, including himself. They are greedy, violent, dirty, avaricious, and destructive of themselves and others. While they are constantly likened to human beings by Gulliver and the Houyhnhnms, an important distinction is drawn: human beings are endowed with reason, and Yahoos are not. The conclusion is not, however, that humans are better than Yahoos, but that they are worse, since humans (unlike Yahoos) have the ability to choose good or evil, and frequently choose evil. The Yahoos are therefore not identical to humans, but symbolize humans at their worst.
Don Pedro de Mendez represents the ideal human being, possessing the best qualities of the Houyhnhnms but also being emotionally warmer and more of an individual than they. He generously helps Gulliver re-adapt to human society. It is significant that Swift made him a member of a Catholic nation at a time when England defined its friends and enemies by whether they shared the Protestant religion. In this detail, Swift shows that such positive human qualities as kindness and charity transcend petty politics.