Constant vigilance: that is the task of the people who protect society from enemies intent on using subterfuge and violence to get their way. It is also the watch word of those who fear that the protectors will pursue the collective interest at untold cost to individual rights. Edward Snowden, a young security contractor, has come down on one side of that tussle by leaking documents showing that the National Security Agency (NSA) spied on millions of Americans’ phone records on the internet activity of hundreds of millions of foreigners.

The documents, published by the Guardian and the Washington Post, include two big secrets. One is a court order telling Verizon, a telecoms company, to hand over “metadata” , such as the duration, direction and location of subscribers’ calls. The other gives some clues about a programme called PRISM, which collects e-mails, files and social-networking data from firms such as Google, Apple and Facebook. Much of this eavesdropping has long been surmised, and none of it is necessarily illegal. America gives wide powers to its law-enforcement and spy agencies. They are overseen by Congress and courts, which issue orders to internet firms.

Barack Obama has responded to the leaks by saying that he “welcomes” a debate on the trade-off between privacy, security and convenience. Despite the president’ s words, however, the administration and much of Congress seem unwilling to talk about the programmes they oversee; and the politicians and executives who do want to speak out are gagged by secrecy laws. Opinion polls show that Americans are divided about the merits of surveillance— which is partly because they know so little about what is going on. But spying in a democracy depends for its legitimacy on informed consent, not blind trust.

You might argue that the spies are doing only what is necessary. Al-Qaeda’ s assaults on September 11th 2001 demonstrated to politicians everywhere that their first duty is to ensure their own citizens’ safety. With Islamist bombers, there is a good case for using electronic surveillance: they come from a population that is still hard for Western security services to penetrate, and they make wide use of mobile phones and the internet. The NSA’ s boss, Keith Alexander, says the ploys revealed by Mr. Snowden have stopped dozens of plots. The burden on society of sweeping up information about them has been modest compared with the wars launched against Afghanistan and Iraq. And the public seems happy: if there were another attack on America, Mr. Snowden would soon be forgotten.

Yet because the spies choose what to reveal about their work, nobody can judge if the cost and intrusion are proportionate to the threat. One concern is the size, scope and cost of the security bureaucracy: some 1. 4 million people have “top secret” clearances of the kind held by Mr. Snowden. Is that sensible?

A second worry is the effect on America’ s ties with other countries. The administration’ s immediate response to the PRISM revelation was that Americans have nothing to fear: it touched only foreigners. That adds insult to injury in countries that count themselves as close American allies: the European Union, in particular, fastidiously protects its citizens’ data. Fears abound that the spy agencies practice a cynical swap, in which each respects the letter of the law protecting the rights of its own people—but lets its allies do the snooping instead.

Lawyerly officials denials of such machinations fail to reassure because of the third worry: the governments acting outside public scrutiny are not to be trusted. James Clapper, America’ s director of national intelligence, told Congress in March that the NSA does not gather data on “millions of Americans” . He now says he answered in “the least untruthful manner” possible. Trawls through big databases may produce interesting clues —but also life-ruining false alarms, especially when the resulting decisions are cloaked in secrecy. Those on “no- fly lists” , which ban an unknown number of people from most air travel, are not told what they have done wrong and cannot clear their names. In desperation, 13 American citizens, including some who were exiled from their own country by the travel ban, are suing the government.

Our point is not that America’ s spies are doing the wrong things, but that the level of public scrutiny is inadequate and so is the right of redress. Without these, officials will be tempted to abuse their powers, because the price of doing so is small. This is particularly true for those who bug and ban.

Spooks do need secrecy, but not on everything, always and everywhere. Officials will complain that disclosure would hinder their efforts in what is already an unfair fight. Yet some operational efficiency is worth sacrificing, because public scrutiny is a condition for popular backing. Even allowing for the need to keep some things clandestine, Americans need a clearer idea of what their spies are doing in their name.