"Tell me where it is!" you scream. "Now!" You know there is little time left. Somewhere in your city, a time bomb is ticking. Whether it spits serin into the air, uranium into the water or atomic fire into the heavens, you do not know.

He does. But he is not talking. Involuntarily, you raise your hand as if to strike. What you are about to do violates the law and your conscience. And yet. ...

In peacetime, torture ranks next to murder as a primal sin. But during war, the debate begins over whether this evil can ever be justified to combat the seemingly greater evil of the enemy. Harvard law Professor Alan Dershowitz has said torture should be legalized.

In early October, the U.S. Senate voted 90-9 to ban it. Although Secretary of State Condoleezza Rice and President Bush have both recently asserted that "We do not torture," five U.S. Army Rangers were charged in November for punching and kicking detainees in Iraq, secret U.S. prisons have caused anxiety in Europe, and Vice President Dick Cheney has battled to win the CIA an exemption from the torture ban. As late as December, the U.S. House of Representatives stood poised to defeat the White House.

Few of us will ever be asked to torture. But, indirectly, all of us have to make a choice: to support, as citizens, those politicians who back torture, or those who seek its prohibition.

The decision of an individual to support, or reject, torture seems at first to be a purely moral question. But what would be the long-term consequences to society if we were to make this radical break with the past?

One cannot do experiments with societies, or predict the future, but, it turns out, one can attempt to address this issue using the cold, hard tools of mathematics and logic. This story begins in 1963.

The United States and the Soviet Union are on the perpetual brink of war, balanced like two sides of an equation. On the American side are "game theorists" like Thomas Schelling, recently awarded the Nobel Prize for his work on the strategy of conflict. On the Soviet side, there is the solitary mathematical psychologist Vladimir Lefebvre.

Just as mathematics could be used to describe logical reasoning, Lefebvre saw that mathematics could be used to describe ethical reasoning. If something was good -- for example, "church," "democracy," "prosperity," "kindness" -- it had value "1."

If something was evil -- "earthquake," "famine," "military defeat," "murder" -- it had value "0." But rarely were ethical situations so simple. For instance, "killing" is bad (0) but protecting one's country is good (1) -- so is war 1 or 0?

Lefebvre saw that, at the crudest level, there were essentially two types of ethical systems. Those that held that employing evil means to attain just ends was good, and those that saw that employing evil means to attain good ends was wrong.

There were also, crudely put, two types of relations between individuals: those entailing compromise (or cooperation) and those entailing confrontation.

Of course, evil people rarely see themselves as evil. So Lefebvre had to incorporate in his model of human nature the capacity of human beings to judge -- correctly or incorrectly -- the goodness or evil of their own acts, and to reflect upon their own judgments, and others'. "Reflexive Theory" was born.

It quickly became a paradigm within the Soviet defense establishment, with the publication of books such as "Mathematics and Armed Conflict." Nothing like it was known in the West.

With very simple assumptions -- for instance, that an individual who correctly sees his actions to be good when they are good, and evil if they are evil, is more highly regarded by society than an individual who incorrectly sees himself -- Lefebvre showed that in a society that accepted the compromise of good with evil, individuals would more often seek the path of confrontation with each other.

Lefebvre's insights were called upon by the State Department during negotiations with Mikhail Gorbachev in Reykjavik, Iceland. (And perhaps Lefebvre's model could be re-enlisted to help U.S. officials understand and negotiate with Arab and Muslim heads of state, who must also negotiate with their people.)

In support of Lefebvre's revolutionary new theory, a survey of Soviet émigrés and Americans was conducted in the 1970s. They were asked questions like, "Should a doctor conceal from a patient that he has cancer in order to diminish his suffering?" Overwhelmingly, the Americans would say no, and overwhelmingly, the Soviets yes. The Soviets accepted the compromise of good with evil; the Americans rejected it.

What does this mean? If Americans begin to accept the use of torture, American society might turn into a society of individuals in conflict.

Not uniformly, thanks to something called free will, but generally, with harmful consequences for society: Imagine two roads, with a stream of cars moving along each one. Each driver wants to reach his destination as quickly as possible; on occasion, drivers will impede each other.

On the first road, drivers rise in their own, and in other drivers', estimation if they yield. Drivers on the second road lose face when they yield. It is clear that traffic will move faster on the first road than on the second.

It can be argued that repressive states like Saudi Arabia, which bred most of the Sept. 11 hijackers, are on the second road. If the United States moved to accept torture, it could veer toward the second road, too -- the road of the Soviet Union.

And we know where that road ends. The Soviet Union no longer exists.

Jonathan David Farley is a mathematician and science fellow at Stanford University's Center for International Security and Cooperation. He has been named one of "15 people who have shaped the global conversation about science in 2005" by Seed Magazine. Contact us at insight@sfchronicle.com.