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.