A number of commentators have used the term “blood libel” to defend conservatives against accusations that right-wing rhetoric may have inflamed Tucson shooter Jared Loughner’s murderous rampage. The term “blood libel” can be traced to an ancient storytelling tradition used by medieval Christians to demonize Jews, the kind of European anti-Semitism that would pave the way for what we now call the Holocaust.

Perhaps the most famous example of a blood-libelous story endures as one of Geoffrey Chaucer's “The Canterbury Tales.” Told by an elegant nun called the Prioress, the story describes the violent and miraculous life of young St. Hugh of Lincoln.

Hugh is a seven-year old Christian boy who learns hymns to the Virgin Mary and sings them on his way to school. The narrow path to the school passes a Jewish ghetto. Tempted by Satan, the Jews are offended by the boy’s song and conspire to murder him.

The details are gruesome and graphic (I’ll translate from the Middle English): “This cursed Jew snatched him and held him tight and cut his throat and cast him in a pit.” This is no ordinary pit: “I say that in a cesspool they threw him where the Jews purge their entrails.” As in the days of Herod and the birth of the Christ Child, an innocent is slaughtered.

In the end, a form of medieval “justice” prevails. The Blessed Mother raises the boy from the dead and restores his beautiful singing voice. The Jews, meanwhile, are rounded up, drawn and quartered by horses, and their remains hung from trees.

Versions of such stories persisted well beyond the Middle Ages. Examples abounded in Nazi propaganda and can still be found anti-Zionist myths taught in corners of the Islamic world.

Jews expressed their outrage by declaring such stories libelous. But they were more than libel; they constituted a blood libel against the Jewish people. Descended from the foul notion that Jews were responsible for the crucifixion of Jesus, blood libel stories claimed that the Jews had the blood of children on their hands.

It is in this historical context that our own political and cultural language can be judged.

In a defense of her own political messages, Sarah Palin insisted that her gun imagery – the targeting of opponents, the use of crosshairs, the imagery of reloading weapons – was not designed to provoke violence. The most powerful weapon, she has argued, is not a gun, but the vote. To suggest that her provocative political language would inspire the kind of violence suffered in Tucson was a blood libel. Her own words should not be taken literally. They included acceptable metaphors, the way any politician might declare a “war” against poverty, drug abuse, or breast cancer.

Here are Palin's words, which came after similar comments in The Wall Street Journal and the work of conservative bloggers:

“Within hours of a tragedy unfolding, journalists and pundits should not manufacture a blood libel that serves only to incite the very hatred and violence they purport to condemn."

In trying to make sense of the use of “blood libel” in the context of the Arizona killings, I turned to the work of Canadian literary critic Northrop Frye, who examined the “motive for metaphor” in his book “The Educated Imagination.”

“As soon as you use associative language, you begin using figures of speech. If you say this talk is dry and dull, you’re using figures associating it with bread and breadknives. There are two main kinds of association, analogy and identity, two things that are like each other and two things that are each other.” [My emphasis.]

He explains that we can say that love is like a red rose, which is called a simile, a powerful figure in its own right. When the poet takes out the “like” she moves from simile to metaphor, a much stronger gesture.

"In other words," I wrote in “The Glamour of Grammar," “The metaphor asserts more power than the simile because the author closes the distance between the two elements of comparison. Being a light to the world is more powerful than being like a light to the world.”

Frye provides a cautionary lesson about metaphorical language: that the differences between the compared elements are as important as the similarities. If I present myself as a “light to the world,” I am asking my audience to see my divine qualities and will not blame them for observing the dissimilarities.

To describe oneself as a victim of blood libel carries with it a certain responsibility for proportionality, that the seriousness of the metaphor must equate in some measure with the experience being described. While the football game between the Steelers and the Ravens has already been compared to a war – and the players to gladiators – we recognize that as traditional and hyperbolic. But I would not call a failed athletic performance an “abortion” or a blowout of one team by another as a “holocaust” or “a virtual Hiroshima.”

As we continue to examine and critique the language of politicians and others in power, let’s hold them accountable, not just for their literal claims, but for their figurative language as well, especially their metaphors.