Swift discriminates between people as they are idealized, people as they are damned, people as they possibly could be, and others as they are. The Houyhnhnms embody the ideal of the rationalists and stoics; the Yahoos illustrate the damning abstraction of sinful and depraved Man; and Pedro de Mendez represents virtue possible to Man. Gulliver, usually quite sane, is misled when we leave him, but he is like most people. Even dullards, occasionally, become obsessed by something or other for a while before lapsing back into their quiet, workaday selves. Eventually, we can imagine that Gulliver will recover and be his former unexciting, gullible self.
Swift uses the technique of making abstractions concrete to show us that super-reasonable horses are impossible and useless models for humans. They have never fallen and therefore have never been redeemed. They are incapable of the Christian virtues that unite passion and reason: Neither they nor the Yahoos are touched by grace or charity. In contrast, the Christian virtues of Pedro de Mendez and the Brobdingnagians (the "least corrupted" of mankind) are possible to humans. These virtues are the result of grace and redemption. Swift does not press this theological point, however. He is, after all, writing a satire, not a religious tract.
