Everybody Knows

Saying that everybody knows is almost never a mistake. The statement isn’t sloppy reasoning. It’s a strategy that aims to cut off discussion or objection, to justify fraud and deception, and to establish truth without evidence.

This isn't my experience; I usually find people to use the expression "everyone X" out of frustration, in situations when they really do feel that everyone X, and can't believe that the other person thinks that not-X is normal or reasonable.

I think you missed an opportunity by only quoting the first verse of Cohen's Everybody Knows, because the second and third verses actually make a good chunk of the point you're trying to make, showing the ironic intent of the first verse as platitudes people use to be at peace with unpleasant things they can't personally change. The second verse presents this thesis:

Everybody knows that the boat is leaking
Everybody knows that the captain lied
Everybody got this broken feeling
Like their father or their dog just died

i.e., the need to avoid an unpleasant truth, as "Everybody talking to their pockets / Everybody wants a box of chocolates / And a long-stem rose... Everybody knows."

Then, the third verse shows the more argumentative and manipulative motte-and-bailey use of the phrase, as he accuses a lover of infidelity, despite the fact that "everybody knows that you love me, baby, everybody knows that you really do; and everybody knows that you've been faithful... ah, give or take a night or two".

i.e. the way people retreat to "but of course there are obvious exceptions... and everybody knows about those, too".

(The rest of the song, though, is kinda weird and I expect that it's stuff that's too culturally specific to the place and time where the song was written. Something something drugs, racism, AIDS, maybe nuclear apocalypse?)

Everybody knows this post belongs in the 2019 Review.

One way to see this is to point out that when Alice tells Bob that everybody knows X, either Bob is asserting X because people act as if they don’t know X, or Bob does not know X. That’s why Alice is telling Bob in the first place.

It could also be that everybody (suitable quantification might be limited to: every student in this course/everyone at this party/every thinker on this site/every co-conspirator of our coup/etc) does in fact know X, but not everybody knows that everybody knows X. Depending on the circumstance of this being pointed out this can be part of creating common knowledge of X. This is related, but not identical to the fourth mode (of self-fulfilling prophecies) you describe. Consider the three statements:

X: "the king is wicked and his servants corrupt"

X': everybody in our conspiracy knows X

X'': it's common knowledge in our conspiracy that X

It's clear that saying X' can't make X true as long as our conspiracy doesn't leak information to the king or his servants. It's also clear that saying X' at a meeting of our whole conspiracy makes X'' true, and that this can be a useful tool for collective action. In fact if X' is not quite true (some people have doubts) saying it can make it true (if our co-conspirators are modal logicians using something like the T axiom).

From a more individualistic view-point such a statement of this form could still contain information if Bob does not know that he knows X (consider Zizek's description of ideology as unknown knowns).

What does the claim that ‘everybody knows’ mean?

I think you point to a valid type of problem with conspiracies of savvy and complicity, but mistakenly paint these as weapons asymmetrically favoring the forces of darkness. Perhaps the Common Knowledge framing makes it clearer, but the modes you describe are degenerate cases of tools for Justice:

The first central mode is ‘this is obviously true because social proof, so I don’t have to actually provide that social proof.’ 

The first mode attempts to look as deferral to experts on a question of fact. This often isn't as useful as discussing the object level, but might be more effective and legible if used as basis for a decision if the statement about expert opinion is not a lie.

The second central mode of ‘everyone knows’ is when it means ‘if you do not know this, or you question it, you are stupid, ignorant and blameworthy.’

We need to be able to punish defection in a legible way so it doesn't degenerate into blame games. For this we need common knowledge about these things so that people don't have excuses for saying or acting along the lines of not-X, but it really needs to be common knowledge so that people don't have excuses for not punishing defection on this point.

A third central mode is ‘if you do not know this (and, often, also claim everyone knows this), you do not count as part of everyone, and therefore are no one. If you wish to be someone, or to avoid becoming no one, know this.’ 

Restricting the universe over which you quantify can be really useful. In particular if you want to coordinate around a concrete project (like building Justice or a spaceship) it's necessary to restrict your notion of 'somebody' to a set that only includes people willing and able to accept certain facts/norms as given.

The fourth central mode is ‘we are establishing this as true, and ideally as unquestionable, so pass that information along as something everyone knows.’ It’s aspirational, a self-fulfilling prophecy. [...]

As mentioned above the self-fulfilling nature can be subtle, but it's not necessarily an ominous prophecy. It could be more along the lines of: "We all band together in accomplishing our goal, so we accomplish it and everyone who took part is greatly rewarded."

Bob does not know X. That’s why Alice is telling Bob in the first place.

Conversational phrases aren't supposed to be interpreted literally. "Everybody knows" never means "literally every single person knows". This is about equivalent to complaining that people say "you're welcome" when the person really wouldn't be welcome under some circumstances.

Don't be the literal Internet guy who thinks this way.

Echoing the other replies so far, I can think of other practical explanations for saying "everybody knows..." that don't fall into your classification.

1) Everybody knows that presenting a fact X to someone who finds X obvious can sometimes give them the impression that you think they're stupid/uninformed/out-of-touch. For instance, the sentence you just read. For another instance, the first few slides of a scientific talk often present basic facts of the field, e.g. "Proteins comprise one or more chains of amino acids, of which there are 20 natural types." Everybody who's a professional biologist/biochemist/bioinformatician/etc. knows this [1]. If you present this information as being even a little bit novel, you look ridiculous. So a common thing to do is to preface such basic statements of fact with "As is well known / As everybody knows / As I'm sure you know / etc." [2]

No bad faith at all! Just a clarification that your statements are meant to help newcomers or outsiders who may not remember such facts as readily as people who work with them every day.

2) I find myself saying "but everybody knows..." to myself or the person I'm talking to when trying to understand puzzling behavior of others. For example, "everybody knows that if trash bags are left outside the dumpster, bears will come and tear everything up, so why do people keep leaving them there?" In this context, the "everybody knows" clause isn't meant as a literal truth but as a seemingly reasonable hypothesis in tension with concrete evidence to the contrary. If everybody has been told, repeatedly, that trash is to be put in the dumpster and not next to it, why do they act like they don't know this? Obviously there is no real mystery here: people do know, they just don't care enough to put in the effort.

But especially in more complex situations, it often helps to lay out a bunch of reasonable hypotheses and then think about why they might not hold. "Everybody knows ..." is a very common type of reasonable hypothesis and so discussion of this sort will often involve good faith uses of the phrase. Put another way: not all statements that look like facts are meant as facts and in particular, many statements are made expressly for the purpose of tearing them down as an exercise in reasoning (essentially, thinking out loud). But if you're not aware of this dynamic, and it's done too implicitly, it might seem like people are speaking in bad faith.

I guess what I'm trying to say in general is: "this statement of fact is too obviously false to be a mistake" has two possible implications: one, as you say, is that the statement was made in bad faith. The other, though, is that it's not a statement of fact. It's a statement intended to do something more so than to say something.

[1] Of course, even such basic facts aren't even strictly true. There are more than 20 natural amino acids if you include all known species, but, as everybody knows, everybody excludes selenocysteine and pyrrolysine in the canonical list.

[2] The alternative is to exclude these first few slides altogether, but this often makes for a too-abrupt start and the non-specialists are more likely to get lost partway through without those initial reminders of what's what.

Besides being a really great object level post, I think it's also an great example of pointing to a subtle conversational move that appears pretty innocuous but upon investigation often is being used to sabotage information flows, intentionally or otherwise. I think a large part of rationality is being able to spot and navigate around these moves.

A version of this that I hear fairly often is “it’s common sense that...”

It works in the same way in that it makes it socially costly to argue against but is more insidious than “everybody knows” (at least in my circles “it’s common sense” has more of a veneer of respectability).

Both also have their proper uses which I think makes the improper uses more difficult to counter.

Another way to respond, rather than showing that it's not a correct statement, is to remember that it doesn't matter. "I don't care what people know, I care what's true".

This will be more effective on some topics than others, but that's true of any discourse tactic.

I... feel like this post is important. But I'm not actually sure how to use it and build around it.

I have vague memories of seeing this link-dropped by folk in the Benquo/Jessicata/Zack/Zvi crowd in various comments, but usually in a way that feels more like an injoke than a substantive point. 

I just checked the three top-level-post pingbacks, and I do think they make meaningful reference to this post. Which I think is sufficient for "yes this concept got followed up on in the past 2 years". But I'm left with a vague frustration with the concept feeling slippery-er than it needs to be.

Maybe one problem is that this post is called "Everybody knows", and not "No, Everybody Doesn't No." So it's harder to namedrop it in a way that solidifies the concept rather than obfuscates it. (Similar to recent troubles with "Alike Minds Think Great", which ended up getting renamed to "Great Minds Might Not Think Alike [LW · GW]")

including 20% who think the Sun revolves around the Earth

The sun does revolve around the Earth. The sun revolving around the earth is equivalent to the Earth revolving around the sun.