Of all the ways in which the world changed after the events of September 11, 2001, one that continues to be fraught is determining the right balance between privacy and security.
Without the possibility of an objectively correct answer, legislators in the US and around the world have sought to find the fulcrum of fairness, or at least of public opinion - the point where the need to prevent terrorist acts is in equilibrium with the danger inherent in shadowy government agencies having evermore awareness of the details of the life of every individual.
In the two months since former National Security Agency contractor Edward Snowden began revealing startling details about the extent of surveillance, this debate on the compromise between security and privacy has intensified, and not only in the US.
Britons have been startled to learn of the extent to which their own security services cooperate with - and are subsidised by - the NSA. Two big German internet service providers said they will encrypt customers' emails. Two US-based providers of secure email, under government pressure to reveal users' data, have closed shop, so as to not "become complicit in crimes against the American people," in the words of one.
President Barack Obama is pushing back against the rising tide of public concern. In remarks on Friday he defended the programmes but promised to make them better understood and more palatable.
Mr Obama seems to be aware that Americans are increasingly wary of blindly trusting big government, even in the name of fighting terrorism, to decide by itself and in secret what information it will acquire about individuals, and how it will use the data.
The president's initiative so far features words rather than actions, and reaction has been mixed. The same day, the Guardian, reporting information from Mr Snowden, revealed that the NSA has, under a rule kept secret from the public, the power to delve more deeply than previously known into American citizens' email and phone calls, all without a warrant.
Revelations like that one will not help Mr Obama's case. At what point does government snooping become so intrusive and affect so many people that it can be considered as dangerous to society as a terrorist attack?
In an age when internet communication is essential to daily life, societies long proud of their liberty are now realising that they face this double-edged danger, and must decide how to respond to it.
