Back in March, the Transportation Security Administration announced it would rescind its longstanding prohibition against the carriage of small knives in airplane cabins. Beginning in mid-April, passengers would be allowed to carry implements with blades of up to 2.36 inches* onto planes.

Well, it's mid-April, and it's not happening. The announcement triggered strong backlash from flight attendant groups, pilot groups, and airlines, and in response the TSA has postponed the changes until further notice.

The concern, of course, is that relaxation of the ban could lead to more violence in the cabin. Flying can be stressful; planes are crowded and tempers occasionally flare—or worse. There have been numerous cases of so-called air rage in which passengers have assaulted flight attendants. The presence of knives, many believe, makes such attacks more likely, and more lethal.

Before exploring the TSA's side of things, let me say up front that I am not endorsing their earlier decision to loosen the rules. Unfortunately this topic is so radioactive that it's difficult for anybody (including me) even to discuss it without being harangued and ridiculed. You don't need to remind me about the benefits of working behind a locked and armored door, and I'm perfectly aware of the threat posed by unstable or intoxicated passengers. Obviously I'm not in favor of a policy that would make it easier for somebody to physically injure a colleague. I'm just telling you what I believe the agency's thinking is.

Basically it follows two lines of reasoning:

The first is knowledge that a deadly sharp object can be easily improvised and fashioned from virtually anything, including all kinds of materials regularly found on airplanes. There are thousands of ways to contrive a weapon that's at least as dangerous a two-inch hobby knife. Apparently the TSA feels there is no longer any point in rummaging through bags to confiscate small knives and scissors when an equally lethal implement can be made from a broken first-class dinner plate, a wine glass, a snapped off shard of plastic, or one of the thousands of pieces of metal cutlery used on planes every day. Easing the rules could free up time and resources, allowing guards to concentrate on more potent threats, including bombs and improvised explosives.

The second line of thinking is more emotionally charged. It requires us to move this entire conversation out from under the framework, and the emotional weight, of September 11.

How so? Conventional wisdom holds that the 9/11 attacks succeeded because 19 hijackers took advantage of a weakness in airport security by smuggling box cutters onto jetliners. But it has been argued that what the men really took advantage of was a weakness in our thinking, and our presumptions of what a hijacking was, and how one would be expected to unfold, based on the decades-long track record of hijackings. In years prior, of course, a hijacking meant a diversion, perhaps to Havana or Beirut, with hostage negotiations and standoffs; crews were accordingly trained in the concept of “passive resistance.” The presence of box cutters on 9/11 was incidental. Any sharp objects would have sufficed, particularly when coupled with the bluff of having a bomb. Their plan relied on the element of surprise, not on weapons. So long as the hijackers didn't chicken out, they were all but guaranteed to succeed.

They didn't chicken out, and it worked. But could it work again? Indeed, flight attendants were the first people murdered on 9/11, and with small blades. But it's also true that the hijack paradigm was changed forever even before the morning of September 11 had ended, when the passengers on United Flight 93 realized what was happening and fought back. Because of the awareness of passengers and crew, together with armed pilots and barricaded cockpits, the chances that a jetliner could again be commandeered using knives is at best remote. This was not the case in 2001, but it is true today.

Those opposed to the changes can counter all of this with a simple and logical premise: there's simply no need to make it easier for a passenger to injure somebody while flying. I agree.

Ultimately, if this issue is to be discussed rationally, it needs to be removed from the 9/11 context and put to a simple question: is there reason to think that allowing small knives on a plane could lead to an increase in violence or stabbings? If the answer to that question is yes, the restrictions should remain in place. If the answer is no, relaxing them is acceptable, for the common good of rationalizing and streamlining airport security.

* If you're wondering where 2.36 inches comes from, that's six centimeters. TSA's liquids and gels restrictions are also based on metric measurements. The maximum container size is not 3 ounces, as is commonly believed. It's 3.4 ounces, also known as 100 milliliters. Those travel-friendly containers you buy at CVS are cheating you out of nearly a half ounce!