Not coincidentally, it was also the year of “fake news,” in which pure fiction masquerading as truth (like posts that claimed Hillary Clinton used a body double and that Pope Francis had endorsed Donald Trump) may have spread wide enough to influence the outcome of the election. Some were certainly deliberate lies spread by right-wing Clinton opponents and all-out profiteers, many in countries outside the United States (and possibly even the Russian government). But framing the issue solely in terms of lying actually underplays and mischaracterizes the grand deception being perpetuated inside the internet’s fun house of mirrors.

To better see this, we must distinguish lying from deception. To lie is to deliberately say what you believe to be false with the intention of deceiving your audience. I can deceive you without lying (silence at a key moment, for example, can be deceptive). And I can lie to you without deceiving. That may be because you are skeptical and don’t believe me, but it may also be because what I say is inadvertently true. Either way, you are lied to but not deceived.

That might suggest that deception occurs when someone is actually caused to believe what is false. “Deception,” as philosophers say, is a “success term.” But that’s only halfway there. Deception can happen even without false belief.

The use of social media to spread political misinformation online is partly just a giant shell game. Propagandists often don’t care whether everyone, or even most people, really believe the specific things they are selling (although it turns out that lots of people always do). They don’t have to get you to actually believe the penny is under the wrong shell. They just have to get you confused enough so that you don’t know what is true. That’s still deception. And it is this kind of deception that dreadful for-profit conspiracy sites like Liberty Writers News have been particularly adept at spreading.

Sure, some percentage of people actually believed the content such sites (for instance, that Hillary Clinton was behind the death of a federal agent). But a far greater number of people came away ever so slightly more doubtful of what is true. They didn’t believe Hillary Clinton ordered a hit, but they didn’t disbelieve it either. It simply became part of the background, one more unsettled question.

It used to be that when someone would say something outrageously false (“the moon landing was faked”) it would be ignored by most folks with the reasoning that “if that was true, I would have heard about it by now.” By that, they meant “heard about from creditable, independent sources.” Filters (primarily, editors) worked to not only weed out the bad, but to make sure the truly extraordinary real news made it to the surface.

The internet has made that reasoning moot.

Many of us are ensconced in our own information bubbles. Few people reject crazy claims based on the fact they hadn’t heard about them before now, because chances are they already have heard about them, or something close to them, from the sites that tend to confirm their biases. That makes them more susceptible to taking fake news seriously.

One reason all this matters is that it perpetuates a feedback loop of deception that is particularly useful to demagogues here and abroad. Deliberate postings invented by entrepreneurs are the manure that make the seeds of doubt and credulity grow. Take the case of Eric Tucker, who tweeted a photo of buses in Austin, Tex., that he thought were being used to bus in marchers protesting Donald Trump’s election. His tweet went viral before it could be debunked. The example is illustrative: softened up by the more outrageous postings and innuendo, ordinary citizens can find themselves ignoring obvious alternative explanations (as Mr. Tucker admits he did) in order to post and share “news” which fits a set of background suspicions and biases.

That in turn gives racist white nationalist and other fringe conspiracy sites — not to mention @realDonaldTrump — more to work with. Their subsequent posts soften more people up, and so it goes. It becomes a cycle where few are deliberately lying, but deception is spiraling ever outward.

A second reason this sort of deception matters is subtler, and concerns our attitude toward evidence and even truth itself. Faced with so much conflicting information, many people are prone to think that everything is biased, everything conflicts, that there is no way to get out of the Library of Babel we find ourselves in, so why try? As Mr. Tucker put it, “I’m also a very busy businessman and I don’t have time to fact-check everything that I put out there, especially when I don’t think it’s going out there for wide consumption.”

This attitude is hardly confined to Mr. Tucker — who among us has not shared posts without fact-checking them? Unfortunately, that doesn’t make it right. Almost everything that we encounter online is being presented to us by for-profit algorithms, and by us, post by post, tweet by tweet. That fact, even more than the spread of fake news, can be its own sort of shell game, one that we are pulling on ourselves.

As the late-19th-century mathematician W. K. Clifford noted in his famous essay, “The Ethics of Belief,” ambivalence about objective evidence is an attitude corrosive of democracy. Clifford ends the essay by imagining someone who has “no time for the long course of study” that would make him competent to judge many questions. Clifford’s response is withering: “Then he should have no time to believe.”

And we might add, tweet.