It would be merciful if, when tragedies such as Tucson's occur, there were a moratorium on sociology. But respites from half-baked explanations, often serving political opportunism, are impossible because of a timeless human craving and a characteristic of many modern minds.

The craving is for banishing randomness and the inexplicable from human experience. Time was, the gods were useful. What is thunder? The gods are angry. Polytheism was explanatory. People postulated causations.

And still do. Hence: The Tucson shooter was (pick your verb) provoked, triggered, unhinged by today's (pick your noun) rhetoric, vitriol, extremism, "climate of hate."

Demystification of the world opened the way for real science, including the social sciences. And for a modern characteristic. And for charlatans.

A characteristic of many contemporary minds is susceptibility to the superstition that all behavior can be traced to some diagnosable frame of mind that is a product of promptings from the social environment. From which flows a political doctrine: Given clever social engineering, society and people can be perfected. This supposedly is the path to progress. It actually is the crux of progressivism. And it is why there is a reflex to blame conservatives first.

Instead, imagine a continuum from the rampages at Columbine and Virginia Tech - the results of individuals' insanities - to the assassinations of Lincoln and the Kennedy brothers, which were clearly connected to the politics of John Wilkes Booth, Lee Harvey Oswald and Sirhan Sirhan, respectively. The two other presidential assassinations also had political colorations.

On July 2, 1881, after four months in office, President James Garfield, who had survived the Civil War battles of Shiloh and Chickamauga, needed a vacation. He was vexed by warring Republican factions - the Stalwarts, who waved the bloody shirt of Civil War memories, and the Half-Breeds, who stressed the emerging issues of industrialization. Walking to Washington's train station, Garfield by chance encountered a disappointed job-seeker. Charles Guiteau drew a pistol, fired two shots and shouted, "I am a Stalwart and Arthur will be president!" On Sept. 19, Garfield died, making Vice President Chester Arthur president. Guiteau was executed, not explained.

On Sept. 6, 1901, President William McKinley, who had survived the battle of Antietam, was shaking hands at a Buffalo exposition when Leon Czolgosz approached, a handkerchief wrapped around his right hand, concealing a gun. Czolgosz, an anarchist, fired two shots. Czolgosz ("I killed the president because he was the enemy of the good people - the good working people. I am not sorry for my crime.") was executed, not explained.

Now we have explainers. They came into vogue with the murder of President Kennedy. They explained why the "real" culprit was not a self-described Marxist who had moved to Moscow, then returned to support Castro. No, the culprit was a "climate of hate" in conservative Dallas, the "paranoid style" of American (conservative) politics or some other national sickness resulting from insufficient liberalism.

Last year, New York Times columnist Charles Blow explained that "the optics must be irritating" to conservatives: Barack Obama is black, Nancy Pelosi is female, Rep. Barney Frank is gay, Rep. Anthony Weiner (an unimportant Democrat, listed to serve Blow's purposes) is Jewish. "It's enough," Blow said, "to make a good old boy go crazy." The Times, which after the Tucson shooting said that "many on the right" are guilty of "demonizing" people and of exploiting "arguments of division," apparently was comfortable with Blow's insinuation that conservatives are misogynistic, homophobic, racist anti-Semites.

On Sunday, the Times explained Tucson: "It is facile and mistaken to attribute this particular madman's act directly to Republicans or Tea Party members. But . . ." The "directly" is priceless.

Three days before Tucson, Howard Dean explained that the Tea Party movement is "the last gasp of the generation that has trouble with diversity." Rising to the challenge of lowering his reputation and the tone of public discourse, Dean smeared Tea Partyers as racists: They oppose Obama's agenda, Obama is African American, ergo . . .

Let us hope that Dean is the last gasp of the generation of liberals whose default position in any argument is to indict opponents as racists. This McCarthyism of the left - devoid of intellectual content, unsupported by data - is a mental tic, not an idea but a tactic for avoiding engagement with ideas. It expresses limitless contempt for the American people, who have reciprocated by reducing liberalism to its current characteristics of electoral weakness and bad sociology.

georgewill@washpost.com