The source: “You Never Know(ism)” by Benjamin H. Friedman and Harvey M. Sapolsky, in Breakthroughs, Spring 2006.

Chaos, enemies, disorder, civil war, terrorism, attackers, and pirates are all out there, waiting to pounce on vulnerable Americans. Who and where? Well, You Never ­Know.

You Never Know is the enemy, write Benjamin H. Friedman, a doctoral student,  and Harvey M. Sapolsky,  a professor of public policy and organization, respectively, at MIT. You Never Know is all powerful. You Never Know can’t be beaten. No number of weapons is sufficient. No threat too preposterous. No enemy too weak. No plot too implausible. You Never ­Know.

Read the latest defense planning document, the Quadrennial Defense Review. The United States now faces a hostile mix of terrorists, national failures, civil insurgencies, missiles, and bloated militaries. America must plan to defeat all of them. You Never ­Know.

Actually, argue the authors, today’s Americans are probably the most secure people in history. The wars in Iraq and Afghanistan do not endanger the American homeland. Terrorists are a threat, but they kill only a fraction of the number of people who die each year from the flu. Their attacks since 2001 have been conventional and local, and most of the terrorists do not live ­nearby.

Predictions of terror attacks understate the complexities of making, transporting, and detonating biological and nuclear weapons. Worries about the theft of ­ready-­made nukes are real but probably exaggerated. Most Soviet weapons were apparently built with components that should have deteriorated by now. Rogue states are fewer. Libya and Iraq are neutralized. The remaining such countries, North Korea, Iran, and Syria, are far ­away.

China may never be able to spend even half of what the United States does on defense. Right now it spends one-­tenth.

Everyone involved in national security focuses on eliminating threats rather than assessing the likelihood of their occurrence. While Friedman and Sapolsky don’t see a deliberate effort to exaggerate the peril, they say media coverage causes the public to develop an exaggerated sense of ­danger.

You Never Knowism is a product of politics. In a democracy, govern­ment expenses require justification. Threats justify budgets, so strategies sell threats. This is not deliberate dishonesty. It is organizational ­culture.

Uncertainty and ignorance are not sufficient grounds for pre­caution and costly defenses. Decisions should weigh the probability of danger, the cost of its realization, and the effectiveness and cost of countermeasures, the authors say. Skep­ticism should be employed, rather than endless dollars, to defend against . . . You Never ­Know.