“Whenever you need a cop, you can never find one,” goes one clichéd refrain.

“Where were the police before the shooting started?” goes another.

It is a classic complaint. It also is an absurd one. Without the kind of frightful surveillance to be found in pulpish works of pseudo-science fiction, or through the use of often illegal wiretaps and other questionable methods, it is usually only by chance that the police learn in advance that a crime will occur and can be there to prevent it.

While prevention is what we expect of law enforcement, however, it apparently is not what we are willing to accept (nor should we, at the expense of our freedoms).

The current cause célèbre is the surveillance by the New York City Police Department of Muslim gatherings – regardless of the nature of those gatherings, including rafting trips by college students. That some of the NYPD’s surveillance takes place outside the confines of its authority only exacerbates matters. New Jersey politicians – including Gov. Chris Christie, Rep. William J. Pascrell Jr., and Newark Mayor Corey Booker – have expressed shock and indignation. There are even calls for a congressional investigation.

It would not surprise us to learn that some of the NYPD’s tactics overstep civil rights bounds. These are unforgivable and inexcusable, and shame on us if we do not object. They must be effectively addressed with alacrity. Indeed, all concerned – and especially the constitutional experts – need to sit down and examine what is being done and how it can be kept on the correct side of the constitutional line.

On the other hand, we would hope that law enforcement at every level has infiltrated and otherwise is keeping tabs through legal means on groups that pose a very real danger to society, such as the white supremacist groups that spawned the likes of a Timothy McVeigh; the radical environmentalist groups that would blow up office buildings; and anti-abortion gangs (laughingly referred to as pro-life advocates) who shoot physicians on their way home from synagogue.

We should expect no less when it comes to the threat posed by radical Islamists. It is racist and untrue to say that every Muslim is a terrorist, or that every terrorist is a Muslim. It is undeniable, however, that Islamic extremists exist, that they often use even the most innocent Muslim gatherings to recruit volunteers for their missions of death and destruction, and that their hatred of all things Western knows no limit.

When an act of terrorism happens here, we demand to know why law enforcement failed to prevent it. Let not the answer ever be because we did not allow law enforcement to do its job.