You may know the Cheney doctrine, as described by Ron Suskind in his book The One Percent Doctrine:

“Even if there’s just a 1 percent chance of the unimaginable coming due, act as if it is a certainty. It’s not about ‘our analysis,’ as Cheney said. It’s about ‘our response.'”

You may also know the Precautionary Principle, a guiding light for some parts of the environmental movement

“When an activity raises threats of harm to the environment or human health, precautionary measures should be taken even if some cause-and-effect relationships are not fully established scientifically.”

But you’ve never seen them both in the same room at the same time, because they’re the same idea: act first, ask questions later.

And, as University of Chicago law professor Cass Sunstein has shown here and here and elsewhere, it’s a bad idea, one that seems to make sense only as long as you selectively apply it. “From the standpoint of decision theory, Vice President Cheney’s remark, and the Precautionary Principle, run into a serious problem: a 1/100 chance of a bad outcome just isn’t equivalent to a certainty of a bad outcome. (You wouldn’t spend the same amount to avoid a 1/100 likelihood of a loss as a 100/100 likelihood of a loss.)”

Both versions are rooted in impatience and frustration.  Cheney has long hated the idea of limits on presidential action.  Environmentalists have long hated the way cost-benefit analyses get skewed to make development projects and new chemicals look benign.

But once you move beyond frustration and take either version of the principle seriously, it collapses. No environmentalist would apply the precautionary principle to the issuance of car fuel-economy regulations, even though they might threaten human health if they result in the production of non-crashworthy vehicles. And as John Allen Paulos says, hopefully no young male Cheneyite would shoot a guy in a bar for giving him a hard stare, just because there was a 1 percent chance that the other guy might shoot him later.

There’s no bumper-sticker way out. In any and all cases, you have to weigh costs and benefits, the possible amount of harm, and the degree of certainty on both sides.  Why doesn’t this go without saying?