The only justification for the ever-expanding powers of the federal government, both in overseas military activities and in domestic surveillance of its own citizens, is in preventing acts of terror. Push any supporter of the war in Iraq and eventually they will say something like "Better to fight them over there than over here." Pester any supporter of the Patriot Act and they will argue that a small, hypothetical invasion of privacy is a small price to pay to foil terrorist plots.

Both camps will point to a range of successes, actual terrorist attempts thwarted by information collected in an unscrupulous manner, whether by torture or Taser, by waterboarding or wiretapping. Indeed both camps are two sides of the same devilish coin, on which is embossed their mutual motto: The ends justify the means.

The ends are to prevent another 9/11. There are basically no limitations on the means, except perhaps on the rate of growth. Like a virus, the means tend to grow slowly, waiting until we are accustomed to one before imposing another. National ID cards, mandatory tracking databases, roving wiretaps, and perhaps even reinstating the draft are all more than just possibilities: They have all been introduced and sometimes passed by Congress.

Why stop there? Let's imagine for a moment that the government has the technology to unobtrusively read our minds. Should we let them do it?

One answer would be to allow it as any search is allowed: with a proper warrant duly issued by an independent judge on the basis of reasonable suspicion of criminal activity. In this case, the technology would have to stand up to rigorous standards. It would be the state's burden to prove that it is effective, accurate, and unlikely to result in false positives (incorrectly classifying innocents as criminals). After all, who among us hasn't had harmless assassination fantasies that end with a glorious return to our libertarian roots? (Dear Secret Service: not me. Assassination is un-libertarian. I fantasize about a non-violent return to freedom. Good times!)

The other answer, the answer of the devilish coin, is to allow mind-reading so long as the data is used only to prevent future terrorist plots and is well-protected. We wouldn't want all that information to fall into the wrong hands. In this case, the technology can even be ineffective, faulty, and biased, so long as it helps stop a crime or two.

If you support the war in Iraq or other preventive wars, or national ID cards, warrantless wiretapping, or any of the other intrusions on our privacies, then you hold in your pocket the devilish coin and its motto is your mantra. To be consistent, you must also endorse a universal application of the government's mind-reading device.

This isn't science fiction. The British Guardian reported last week that scientists have developed a method using MRI scans that can accurately predict the images people are thinking of. In a test where random guessing would have resulted in less than one correct prediction out of a hundred, their method correctly predicted ninety.

Of course, requiring all Americans to lie in MRI machines for hours at a time is still a few years off. (First we need to be issued national ID cards with magnetic strips to facilitate processing.) But there is actually a better and simpler technology already available. Why not let the government listen to your phone calls, read your emails, and track your web traffic?

It wouldn't even be very invasive. No human agents even need to be involved unless reasonable red flags appear, like if the word assassination appears anywhere near Patriot Act or Iraq or libertarian. (Are you reading the online version of this article? Wait for a knock on your door.)

If you don't like this scenario, ask yourself where you would draw the line. Words like reasonable are inappropriate here, because the government, and the people, could decide that all of this is reasonable, but that still wouldn't make it justifiable.

There is no fuzzy gray line here. The government should not initiate force against innocent people. Period. (Neither should anyone else—that's all of libertarianism in a nutshell.)

The devilish coin should be tossed—into the wastebasket.