Expression for (wrong) 'common knowledge'. A set of common misconceptions

Question

When some knowledge is available to most of us, quite evident and widely accepted, we say it's 'common knowledge'.

Let's think about the same situation, except that this so-called knowledge is proven wrong, unfounded from the very beginning, or simple dogma.

'Common rhetoric' seems more like it: things people say all the time despite being wrong. But it implies that there's an outspoken support of those ideas and goes beyond the scope of the expression I'm after.

What should I call that 'common mindset', that erroneous 'usual way of thinking'?

e.g.

#1

-I'm reluctant to ask Sally to prom.

-And I can see why; the common rhetoric is you should go with Janet.

#2

The common rhetoric in the GOP is that tax cuts for the rich stimulate the economy so much that they pay for themselves. This unfounded, nonsensical claim has received the appropriate name of voodoo economics.

Answer 1

A myth or an urban legend sprang to mind on reading the title.

For the particular case, you could consider (common) misconception.

I must admit I don't fully understand exactly what you're trying to express, mainly because of your use of the word rhetoric in the example. Please provide some clarification

Answer 2

fallacy

A fallacy is the use of poor, or invalid, reasoning for the construction of an argument. It is also used to refer to "an argument which appears to be correct but is not." If an argument is fallacious, it does not necessarily mean the conclusion is false. Fallacies are commonly divided into those that are formal and those that are informal. A formal fallacy can neatly be expressed in standard system of logic; for example, propositional logic. Conversely, an informal fallacy originates in an other error in reasoning than an improper logical form. Arguments committing informal fallacies may be formally valid but still be fallacious. Fallacies of presumption fail to prove the conclusion by assuming the conclusion in the proof. Fallacies of weak inference fail to prove the conclusion due to insufficient evidence. Fallacies of distraction fail to prove the conclusion due to irrelevant evidence, like emotion. Fallacies of ambiguity fail to prove the conclusion due to vagueness in words, phrases, or grammar. Some fallacies are committed intentionally (to manipulate or persuade by deception), others unintentionally due to carelessness or ignorance.

A fallacy fits your requirements, it implies faulty reasoning but not necessarily that there's outspoken support for it.

Answer 3

Try:

Popular misconception, alleged fact, distorted claim, common claim, false allegation, purported truth, brainwashing, propaganda, received truth, disinformation, misinformation, outmoded notion, widely-held notion, false idea, mistaken idea, mistaken notion, communal lie, media lie, mass myth, usual rhetoric...

These are just some of the many possible permutations that might fit your needs, depending on the exact situation or circumstances being described. This is a rich topic for mixing and matching the various possible elements.

Answer 4

I must protest against "the common rhetoric is you should go with Janet." The word does not mean anything like what you think it does, and nobody would say this. Please bury this one with stake through its heart.

Not fallacies, these things don't arise from invalid syllogisms.

Here, people make me shut my windows at night, lest the local sorcerers take the shape of owls and fly in. This is "common knowledge", although of course no one has ever seen one doing it. Your mother told you when you were five, so you believe it the rest of your life, and "Everybody knows". I can't think of anything better than "superstition" or "delusion(al system)".