Zvi's Law of No Evidence

I agree with you that Zvi and Eliezer are both correct, but I am unconvinced by your analysis of why. I don't think "no evidence" is vague. I think it's false.

"There is no evidence of Bigfoot" is simply untrue. There are people who claim to have seen things-that-maybe-could-be-Bigfoot. There are people who claim to have found footprints and the like. All these things are evidence. Crappy evidence, which is why it doesn't do much to change my opinion that Bigfoot is almost certainly not real, but still evidence.

"Nobody looked for Bigfoot and nobody found him" is also evidence that Bigfoot doesn't exist. (There are possible worlds in which Bigfoot gets seen by people who weren't actively looking. If nobody looked and nobody found, those worlds are ruled out.)

So, when someone says "there is no evidence of Bigfoot", what they are saying is not literally true, and they almost certainly know it, and that's evidence of bullshit or, at best, thinking sloppy enough that they might as well be bullshitting.

We can get back to something like your complaint of vagueness if we apply a bit of extra charity and suppose that when they say "there is no evidence of Bigfoot" they mean something like "there are no pieces of evidence of Bigfoot that are individually very strong evidence". In that case, there are two possible charges of vagueness we could bring:

• It could mean "there are no photos that are unquestionably of Bigfoot", or it could mean "there are no credible eyewitness reports", or it could mean "there are no videos", etc. I don't really buy this criticism; I think it's fairly clear that the meaning is pretty much what I said above.
• Having decided what it means, it then potentially matters a lot what things have been going on that one might expect to produce such evidence. As you say, having no photos, eyewitness reports, etc., is stronger evidence if lots of people have been out looking hard than if no one has been.
  • But I don't think this situation is accurately described by saying that the statement is vague. It's just a fact of life that sometimes the conclusions we should draw from an observation depend on its context.
  • If someone says, in so many words, "To the best of my knowledge, no one has ever presented any piece of evidence for Bigfoot that a reasonable person would find compelling", I don't think I could reasonably complain that they're being vague or bullshitty, even though what I'd conclude would depend on how hard I thought people had been looking for such evidence.

This misses the point of Zvi's comment. Alice saying "there is no evidence of X" is more likely in worlds where Alice is BSing than in worlds where Alice is attempting to provide factual information (ie Level 1 communication). That is orthogonal to any actual calculations of {amount of evidence observed} / {amount of evidence that could have been observed, given the amount of looking that has been done}.

Also, too, the second thing is a gradient, not a dichotomy. And "your priors" are just a way of saying {amount of evidence observed that I know about} / {amount of evidence that could have been observed, given what I know about the amount of looking that has been done}

Absence of evidence of X is evidence of absence of X.

A claim about the absence of evidence of X is evidence of:

• the speaker's belief of the listeners' belief in X,
• absence of evidence of NOT X,
• the speaker's intention of changing the listeners' belief in X.

No paradox to resolve here.

I appreciate you trying to write this up, but as other commenters have noted, there's no contradiction here in the first place and you appear to have missed the point.

As far as I can tell, if you understand Yudkowsky's point, Zvi's follows directly. {No evidence of X} = {evidence of not-X}, but the speech act of claiming "There is no evidence of X" only occurs when there is some evidence worth claiming doesn't count as evidence.

And Yudkowsky's point also points out that essentially "no evidence" is not just vague but in virtually all cases just completely misleading. It would be better to say "the absence of any photographs or eyewitness accounts is evidence that this story was fabricated."

There is evidence for and evidence against but there is no "no evidence".

Well, apparently I decided to write this up as its own post [LW · GW].

If someone says there is "no evidence" of something then it is because they are trying to pass off "nobody looked for Bigfoot and nobody found him" as "explorers looked for Bigfoot and nobody found him".

A "no evidence" argument doesn't have to be made in bad faith. It's claiming that we've looked into the people who said they saw Bigfoot (as opposed to looking for Bigfoot itself), and concluded those claims have no good evidence behind them. And so, without evidence, we should rule out Bigfoot, because the prior for Bigfoot is very low. We would need positive evidence to raise the Bigfoot hypothesis to the level of conscious consideration, and we claim there is no such evidence.

Yes, a claim of "no evidence" is - in this context - a social attack on the people who were talking about the subject (and so implicitly claiming "yes evidence"). In the highly politicized context Zvi is discussing, almost all factual arguments are disguised social attacks; rhetorics, meant to persuade people, with facts and logic being instrumental but not the goal.

And so we can justly ignore the whole discussion because we think it's not about facts and arguments and real "evidence" and it never was. But if we want to engage with the discussion using our own arguments and evidence (or to pretend to do so for our own social goals), then we should acknowledge that a valid factual claim is being made here, which we can evaluate without dismissing it as purely rhetorical manipulation ("passing off argument A as argument B").

Zvi wrote,

No evidence should be fully up there with “government denial” or “I didn’t do it, no one saw me do it, there’s no way they can prove anything.” If there was indeed no evidence, there’d be no need to claim there was no evidence, and this is usually a move to categorize the evidence as illegitimate and irrelevant because it doesn’t fit today’s preferred form of scientism.

I disagree with this. If people claim Bigfoot exists, and I think they have no evidence for that claim, then yes I will say there is no evidence. The mere fact that people claim A is not in itself evidence for A, because people are not pure truth-seekers, and if I acknowledge any claim as itself constituting evidence, they will proceed to claim lots of things without evidence behind them. I don't need to "categorize the evidence as illegitimate and irrelevant", I should be able to say plainly that there is no evidence to begin with. It's not because "it doesn't fit today's preferred form of scientism", it's because seeing a vague outline in a snow-storm really truly isn't evidence for Bigfoot.

When people we don't like claim things that are clearly wrong, we may want to dismiss their arguments are rhetorically invalid or malicious or made in bad faith. To claim that the form of such arguments necessarily indicates they are being made in bad faith. But that is engaging on their terms - analyzing why they're making the arguments, instead of analyzing the arguments themselves (simulacra levels!). These two discussions are both necessary but they should be kept apart. On the object level, we should be able to keep saying - the arguments are not "wrongly shaped", they are just factually wrong.

Isn't this the Heap paradox? Enough absence of evidence probably should be considered to be evidence of absence, but it's not obvious where that cutoff should be, and it can vary from one situation to the next.