Whatever goes upon two legs is an enemy. Whatever goes upon four legs, or has wings, is a friend.

First proclaimed by Old Major in his final address to the animals of Manor Farm, the dichotomy "Whatever goes upon two legs is an enemy. Whatever goes upon four legs, or has wings, is a friend" became the foundational doctrine of Animalism. Major argued that all human beings—the sole consumers who produce nothing—were the enemy, and that every non-human animal was a comrade. This binary cut the world into two mutually exclusive categories, enforcing a total separation between the oppressors and the oppressed.

After the Rebellion, the two clauses were inscribed as the first and second of the Seven Commandments on the wall of the big barn, intended as an unalterable law. The pigs, led by Snowball, soon simplified the commandments into a single maxim: "Four legs good, two legs bad." The sheep took up the chant with such enthusiasm that it became a ubiquitous slogan, used to drown out dissent. Snowball also had to persuade the birds that their wings counted as legs, ensuring that the principle of "four legs or wings" covered all animals—a casuistic reinterpretation that foreshadowed later manipulations.

As Napoleon's tyranny tightened, the other commandments were quietly amended: the Sixth gained "without cause," the Fifth "to excess," the Fourth "with sheets." But the binary itself remained untouched until the pigs themselves began to walk on their hind legs. Squealer was the first to appear upright in the yard, followed by a file of pigs, and finally Napoleon himself, majestically erect, a whip in his trotter. The creature that had defined the enemy now embodied the very form of the enemy.

The final inversion came when the sheep, after a week of secret training with Squealer, burst into a new bleating: "Four legs good, two legs better!" The slogan had been rewritten to endorse the pigs' human-like posture. At the novel's end, the pigs and human farmers sit together, drinking and playing cards, their faces indistinguishable. The original classification is destroyed: the pigs have become the two-legged enemy they were founded to oppose. The expression's journey from revolutionary principle to hollow dogma mirrors the entire corruption of the Animal Farm ideal.