In the "beast fable," entitled "The Trial of Reynard," a fox (Reynard) is guilty of many crimes. Captured, he is sentenced to die, so through deceit he makes up a story to save himself. He asks King Nobel if he can confess publicly, and then concocts a story about a plan hatched by the other animals to kill the King. Reynard stopped them.
To convince the King that his story is true, he says he would surely have died from hunger—cheated by the other animals of his share of the "plunder" in their nefarious crimes—if he didn't have a treasure hidden away:
'Secretly have I fed well by means of that excellent treasure,
All of silver and gold in a secret place that securely
Hidden I keep; with this I’ve enough. And, I say it in earnest,
Not a wagon could carry it off, though sevenfold loaded.’
When the King hears about the money, he wants details—which are more of Reynard's lies. The fox says that he devoted himself to saving the King and Queen's lives. The Queen is very taken with the fox's tale and invites him down from the gallows; he explains he stole the gold from the villains that had planned the King's death—money that was to pay for "assassins." The Queen is touched by the story and pleads for Reynard's pardon. Reynard promises to give the "treasure" to the King, so his life is spared.
In Sir Gawain and the Green Knight, deception is also used: however here it is not used for selfish reasons—as is seen in the tale of Reynard—but to test the honor—the chivalry—of one of King Arthur's knights of Camelot.
It was Arthur's custom not to eat breakfast before an adventure had presented itself at the castle each day. It is at Christmas and sure enough, a green knight presents himself, offering anyone a free swing of the axe to remove his head, if he can "return the favor." Young Gawain agrees. He cuts off the knight's head...and the knight stands, retrieves his head, and reminds Gawain of his promise to meet the magical knight in one year's time.
A year later, Gawain travels to meet his destiny. Arriving early, he discovers a castle. The host, Bertilak, and his lady wife invite Gawain to spend the holiday with them. Each day Bertilak hunts and promises to share what he earns that day with Gawain, expecting Gawain to do the same. Lady Bertilak tries to seduce the knight, but he resists. When Bertilak returns each day to present his kill, Gawain gives Bertilak the kiss collected from the Lady. The last day she also gives him a magic belt to protect him from death—this he keeps a secret.
At the chapel, the Green Knight shows his true self—he is Gawain's host. To test the knight, Bertilak told his wife to try to seduce Gawain, but she failed. He also knows about the belt. The Green Knight takes his three swings of the axe Gawain had agreed to, but only nicks his neck.
Bertilak knows that Gawain was mortified because of his lie as he faced the Green Knight.
An honest man
Need never fear.
But still, the third day, there
In my castle, you failed—and you felt that here. (195-198)
Bertilak says his deceit proved how honorable Gawain was. He understands the young knight's wish to live, and so forgives Gawain for his failure "share the belt." Still, Gawain is shamed by his deceit and learns a hard lesson that honor is more important than escaping death.
Deceit is used in both stories, but for totally different reasons.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.