Swift was often called a misanthrope, someone who despises humanity. (In the critical overview linked below, you'll find plenty of evidence to this postulation.) Whether Swift truly was misanthropic or just sarcastic, he makes his point about the farce of human superiority brilliantly in the fourth part of Gulliver's Travels.
Swift chose horses, beast of burden, to be his rational and logical heroes in this section, while creating the humanoid creature to be as loathsome as imaginable. As Gulliver tries to explain English civilization to the Houyhnhyms, he discovers that his countrymen have nothing of which to boast. The Houyhnymns cannot fathom the concept of dishonesty or greed or corruption or war---or love. They are purely creatures of the Enlightenment at its most extreme: reason, logic, and fact are the only important things.
The Yahoos, boorish, savage, and more animal than human, startle the readers as much as they did Gulliver. In them we see ourselves, stripped of culture and taken down to the basest of instincts. Swift uses the two extremes to point out that man is not as superior as he thinks he is, and that there is something to be gained from the philosophies of reason.
However, Swift tempers his view by making Gulliver a true misanthrope at the end of the book, a man who is unable to enjoy wife and family, and left alone with his horses. Swift recognized that even reason has its limitations. As a clergyman, Swift understood that humanity has great capacity for and a tremendous need for relationship. It is the single flaw of the Houynhnymns: in all their rationality, there is no relationship, no humor, no sorrow, no joy. Swift understood that a truly happy life requires both reason and relationship.