What is the main idea of Gulliver's Travels?
The main idea behind Gulliver's Travels is to persuade Britons to reform their own society. Swift uses his gullible narrator, appropriately named Gulliver, to show through his eyes a number of comically cruel and absurd fictional cultures. Not surprisingly, the beliefs and practices of these societies, if exaggerated, are similar to common beliefs and practices in Britain. Swift point is to satirize the cruelty and irrationality in the Great Britain of his day.
For example, the Lilliputians Gulliver meets are tiny and pretty, but petty and cruel, while the Brobdingnagians are huge and ugly to Gulliver, but kind and humane. Here Swift satirizes the tendency to assign positive moral virtues to people who are outwardly beautiful and condemn those who are ugly rather than seeing through to their souls.
Gulliver finds himself witnessing all sorts of absurdities in his travels. At the Grand Academy in Lagado, he sees money and resources wasted on pointless and sometimes cruel experiments, a satire of the British Royal Academy. He later encounters the Houyhnhnms, intelligent talking horses. They are far more sensible and humane than their human counterparts, the Yahoos—or for that matter, Gulliver's fellow countrymen.
By getting people to laugh at the evils and absurdities of the peoples Gulliver meets on his travels, he hopes also to get readers to dwell on their own evils and their own unquestioning acceptance of cruel customs—and perhaps change their ways.
It is difficult to narrow down the scope of this satire to just one idea, but, broadly speaking, we could say that the major theme of Gulliver's Travels is that human beings are inherently flawed. Though we are capable of some good, often our xenophobia, pride, and lack of humanity in many situations leads us to behave selfishly and without compassion for others. The purpose of satire, however, is not simply to point out someone's flaws; it is to point out someone's flaws in such a way that it will open their eyes to their mistakes and prompt them to change their behavior. Therefore, Swift doesn't simply want to point out our stupidity or our ignorance, he wants to compel us to reevaluate the way we think about many things: religion, war, logic and emotion, immortality, and the list goes on and on.