WASHINGTON — Even those who believe the National Security Agency’s vacuum-cleaner surveillance of electronic communications does not trample privacy rights should be troubled by this practical implication: If you try to know everything, you end up knowing nothing.

An investigation by The Washington Post, which examined a cache of intelligence reports provided by fugitive NSA contractor Edward Snowden, illustrates my point. The agency is gathering and warehousing enormous amounts of private information, most of it irrelevant because it concerns innocent individuals — mostly foreigners, but some U.S. citizens as well.

By “innocent,” I mean the NSA is convinced these people have no involvement with any activity that poses a threat. But the agency keeps their information anyway.

Reporter Barton Gellman, researcher Julie Tate and security consultant Ashkan Soltani spent four months analyzing more than 160,000 text messages, emails, social network exchanges and other communications sucked in by the NSA’s programs. The material Snowden provided was associated with roughly 11,400 individuals, perhaps half of them Americans. Of those whose lives were rummaged through, only 11 percent are identified by the NSA as targets who warrant surveillance. This means that about nine out of 10 just happened to be snagged in the NSA’s net.

It is important to clarify that I’m not talking about content-free metadata, such as the NSA’s controversial log of domestic calls. These are actual emails, texts, conversations and social media posts that the NSA gathered under programs known as PRISM and Upstream. The Post reported that the content includes “stories of love and heartbreak, illicit sexual liaisons, mental-health crises, political and religious conversions, financial anxieties and disappointed hopes.”

It’s also important to note that the 11 percent deemed legitimate targets included some people I definitely want our spies to be watching. The Post said the surveillance records it examined included information that led to the captures of a Pakistan-based terrorist bomb-maker and a suspect in an Indonesian bomb attack. Other successes were withheld by government officials’ request. Perhaps these cases could have been cracked by means other than mass surveillance. Perhaps not.

My point is not that this surveillance is incapable of helping authorities apprehend genuine terrorists. I just believe the snooping should be more targeted — and clearly irrelevant information about innocent people should be quickly erased.

The NSA seeks something like omniscience regarding electronic communications. But it is not enough to have a crucial tidbit of information stored on a server somewhere. For that information to be useful, it has to be identifiable and accessible. The more indiscriminately you amass data, the harder it is to find the relevant bits.

The NSA’s position is essentially that the bigger the haystack it can gather, the more needles it can find. But given the ever-increasing volume of electronic communications around the world, what sense does it make for the NSA to clutter its data banks with information about people — foreign and domestic — who pose no threat? Retaining this material, apparently for up to five years, is not just an invasion of the targets’ privacy but also a waste of the NSA’s capacity for storage and analysis.

If NSA officials are so confident they can manage the unimaginably vast quantities of data the agency is assembling, then why have they repeatedly given public assurances that Snowden — whom they frequently describe as a lowly analyst — had no access to the kind of sensitive data he gave to the Post? Does the agency really have any idea of what is already in its databases? Does the NSA know who might be sifting through this material? And for what purposes?

These NSA programs are designed to snoop on foreigners. Snowden has expressed the view that citizens of other countries have privacy rights, too. You don’t have to agree with him to wonder why the emails of, say, a German college professor or an insurance salesman in Brazil should not be purged once the material is determined irrelevant.

Snowden also believes there are threats. He gave the information to the Post with the understanding that nothing would be published that could harm U.S. national security or endanger lives. Indeed, the paper withheld details about “a secret overseas nuclear project, double-dealing by an ostensible ally, a military calamity that befell an unfriendly power, and the identities of aggressive intruders into U.S. computer networks.”

Investigate that stuff, NSA. Stop wasting time and effort on people who mean us no harm.

Eugene Robinson’s email address is eugenerobinson@washpost.com.