My recent observations on Hoover’s Defining Ideas about the relationship of civil liberties to national security have drawn a stern response from Cato’s own Jim Harper, whose central claim is that I have sounded “needless anti‐​privacy notes” in my attack on the privacy protective policies that have been championed by Massachusetts Republican State Senator Robert Hedlund, whom I criticized for being too squeamish on aggressive and targeted government action to counter the threats that became all too visible on April 15, 2013.

Harper’s initial parry is to stress a proposition that no one should care to deny, namely, that the Fourth Amendment imposes a bar against unreasonable searches and seizures, which in turn requires an examination of the purported relationship between the restriction that government seeks to impose and the evil that it seeks to defend against. But in his choice of example and articulation of principle, Harper is guilty of grievous non sequiturs that add needless confusion to a problem that is already difficult enough to handle.

To examine the relationship between privacy and security, it is always a mistake to start with an example that the author describes as “an illustration ad absurdum,” which is just what Harper does when he bravely denounces a rule that allows for “100% crotch checks at street corners in major cities.” The simple response is that this kind of action is under current law regarded as per se illegal even in connection with the so‐​called Terry stopswhich allow a police officer “to stop and frisk” individual on the public street if he or she has “reasonable suspicion” to think that the targeted person has engaged in illegal activity.

That example has absolutely nothing to do with the design of a workable surveillance system. It also falsely calibrates the relevant choices by dismissing the current cries for increased surveillance as a “closer” question, when the two situations are worlds apart. The Fourth Amendment treatment of unreasonable searches and seizures rests on a critical distinction between investigation of particular suspects and the stopping of dangers from unknown quarters. There is a lot more information in the first case, so that a dragnet search makes no sense, which is why particularized evidence is required. But general surveillance at unknown targets has to spread its net far wider. It is both less intrusive and more comprehensive, and it can and does work. It was painfully clear from the pattern of events in Boston that the private surveillance cameras that were trained on the Boston Marathon provided indispensable information toward identifying and apprehending the Tsarnaev brothers. What makes their use unreasonable, when there is not the slightest evidence that the information so acquired was used for improper purposes unrelated to the search?

It may be “worth discussing,” as Harper suggests, whether the use of surveillance will help deter some crimes and stop others. But, if so, the only useful discussion is one that asks the means‐​ends question of how, in light of cost and privacy concerns, one can construct the best cost‐​effective surveillance system available, which can then be coordinated with the activities of police officers and volunteers on the ground, especially at any public event that presents a soft target.

But to dismiss these efforts on the unsupported speculation that “the possibility of apprehension seems not have occurred to the Tsarnaev brothers” can only be described as blinding error, especially in light of their frantic efforts to escape capture so they could strike again. Nor does it make the slightest sense to tie general surveillance policy to some dubious account of the psychological make‐​up of two individuals. It is far wiser to develop policies that improve the ability to track and identify dangerous suspects. Of course it is possible to construct a “surveillance architecture” that so dense as to be useless. But once again, the sensible case for beefing up Boston’s public surveillance does not require that system designers leap from one indispensable extreme to another. The real question is how to identify the comprehensive policies that do make sense.

Harper is equally off target about the potential gains from racial or ethnic profiling. No one accepts the extreme proposition that all terrorists come from the same ethnic stock or practice the same religion. But that observation offers absolutely no reason to ignore valuable information that could help tweak the design of surveillance systems of searches. The question here is not whether sensible protocols and profiles can narrow the search down to one‐​fifth the world’s population, most of which does not live in Boston anyhow. It is the question of whether one can winnow the list of potential suspects from 100 to 20 people, which, if done reliably, gives law enforcement a huge leg up in conducting its investigations.

In sum, Harper would have a stronger case if he had tried to comment constructively on serious proposals that are put forward. But to take an ill‐​advised a priori position that does nothing to advance either the protection of human life and human property, both private and public, is inconsistent with any sound libertarian position. Remember that libertarians like myself, and I hope Harper, regard the protection of both as the primary function of the state. Harper’s careless and imprecise invocation of the Fourth Amendment cannot conceal this fundamental truth.