Gulliver's description of the horses, the Houyhnhnms, is almost idyllic: "The behaviour of these animals was . . . orderly and rational . . . acute and judicious." Indeed, it is a horse that rescues him from the Yahoos — not by any overt, physical action, but by simply appearing on the road — no physical action being necessary.
Houyhnhnms live simple lives wholly devoted to reason. They speak clearly, they act justly, and they have simple laws. Each Houyhnhnm knows what is right and acts accordingly. They are untroubled by greed, politics, or lust. They live a life of cleanliness and exist in peace and serenity. They live by the grand maxim: Cultivate Reason and be totally governed by it. So perfect is their society, in fact, that they have no concept of a lie, and therefore no word to express it. The only word for evil is "Yahoo."
Swift defines Houyhnhnm as meaning "perfection of nature." This definition establishes an important distinction. The horses are uncorrupted by passion — either base or noble. They are devoid, for example, of charity. Also, they are not subject to temptation. Swift, however, never suggests that the Houyhnhnms stand for perfected human nature; on the contrary, they manifest innocent human nature. What they do — and what they say and think — is akin to human nature, but the character of the Houyhnhnms is far from Gulliver's. They are ignorant of many things which most people would consider venial. They cannot, for example, understand lying — or even the necessity for lying.
Swift thus establishes a range, or spectrum, of existence. The horses are literally innocent, having never (in theological terms) "fallen"; the Yahoos are super-sensual and seem depraved. The Houyhnhnms are ice-cold reason; the Yahoos are fiery sensuality. In between these extremes is Gulliver.