The Yahoos, separated from civilization, lose their ability to act or think rationally, causing them to become violent and greedy. Swift is showing that without reason, a man becomes a slave to his cravings and desires. The Yahoos hate one another more than any other species, causing them to fight over food and material objects, like large stones. Through this fantastic character, Swift is satirizing mankind's folly over material objects. He is showing that when man doesn't exercise reason, he will naturally take an object with little usefulness, place great value on it, causing him to lust after it; many times culminating with the use of violence, like going to war.
The Houyhnhnms serve as an illustration of what happens to man when he is situated on the other end of the spectrum, lacking in emotion. The Houyhnhnms are rational horses whose actions are governed by total reason. Although it is hard to tell sometimes, Swift does not take these characters seriously; he uses them to satirize the vice man has with pride. The Houyhnhnms, whose name suggests the perfection of nature, have a simple government, their own language, and their two guiding virtues are friendship and benevolence. They live in a peaceful society, where they all act right, because being devoid of all emotion they can not act wrong. They know nothing of love, hate, jealously, or lying and false representation. They live in a utopian society, where every horse does not what is good for the individual, but what is good for the common good. Throughout his narrative, Swift is showing us that the Houyhnhnms are indeed smart, but instead of using that knowledge for the benefit of society, they use it for their own prideful glorification. With great effectiveness, Swift uses the Houyhnhnms to satirize the belief that a man of pure rationality is the only complete man. He shows you that this is an ideal gone wrong; a man who relies purely on reason becomes filled with pride, blinding him, and rendering him unable to accomplish what he wants to do.
