1. Mick West

    Mick West Administrator Staff Member

    After discussing a conspiracy theory (such as the "chemtrails" conspiracy theory) for several years online, as I have done, you get very familiar with all the claims of evidence. You also get familiar with the science, facts, and reasoning that debunk those claims of evidence. After running through all these claims with the more reasonable conspiracy theorist (i.e. one who actually listen to you) you quite often arrive at the point where they say "well, that does not prove it's not happening".

    Basically they accept (somewhat) that their claims of evidence don't really hold up, but despite that they assert that this lack of evidence is not an indication that the conspiracy theory is not a real thing. Just because contrails naturally can persist for hours, they say, it does not mean the government is not secretly spraying us to change the climate.

    What this boils down to is the popular saying "absence of evidence is not evidence of absence". This seems quite sensible, and indeed might be quite attractive to debunkers and skeptics because it was popularized by a hero to many skeptics: Carl Sagan. Indeed, many memes exist with the quote, and Sagan's name or image.

    At its simplest level, the quote is very reasonable. The simple fact that you've not seen any evidence for something does not mean that such a thing does not exist. If you've never seen a black swan (as nobody in Europe had in the 16th century) then that's not proof that black swan's do not exist. And in fact their discovery in Australia serves as an excellent example that seems to validate the saying.
    Image Source: Wikipedia, Kiril Krastev

    So why then does this not hold true for conspiracy theories? The answer is that sometimes it does, and sometimes it does not. Sometimes absence of evidence is not evidence of absence, and sometimes absence of evidence is evidence of absence. The dividing line is not a clear one, and the degree to which absence is indicated is based on how likely the presence of a thing would be to create noticeable evidence.

    This can be said two ways:
    • Absence of evidence is not evidence of absence if the existence of a thing is not likely to create evidence
    • Absence of evidence is evidence of absence if the existence of a things is likely to create evidence.
    This is the same thing said two ways. But the important thing to focus on in the context of conspiracy theories is the second one - the case when Sagan's quote is incorrect, and when absence of evidence actually is evidence of absence. And it's useful to pause to note that this is not a novel concept in logic, in fact it's quite well known, and eminently reasonable, essentially inarguable.


    As a scientific skeptic and a debunker, I've largely focussed on specific claims of evidence. For example a claim is made that contrails cannot persist more than a minute, and I debunk this by showing historical books on the weather that explain that contrails can actually persist for hours. This negates the claim of evidence, but does not seem to the conspiracy theorist to disprove their theory (assuming they accept the refutation of the "contrail persistence" claim, which many do not). No matter how many times they bring up new claims of evidence, and you explain why they are wrong, they can fall back on the "absence of evidence is not evidence of absence", and so I think we actually need to point out that, actually yes, it is.

    Let's say your theory is that the government is spraying chemicals into the exhaust of planes, making unusual trails in the sky. The claim here boils down to "some trails behind planes are not water". This is a claim where we would expect to see evidence. If the trails were not just water then they would look and act differently to water. They would have a different refractive index (causing a different sized halo around the sun). If they were something like a solid powder, then they would dissipate like smoke. If they had a higher vaporization point than the exhaust temperature then there would not be a gap behind the engine. It might persist at low altitudes in areas of high temperature or low humidity

    So if none of these things occur, then it is quite reasonable to say that this is evidence that chemicals are not being sprayed into the exhaust of planes. At the very least it would severely restrict the possible set of things being sprayed to things that are very like water - which, if you consider all the ways it is similar to water, is essentially nothing.

    So (again from my perspective as a debunker), there are two important points that follow from this.

    Firstly, while this is evidence of absence, it is not always proof of absence. Instead it severely limits the form that conspiracy theory can take. If you accept the evidence that the trails behave as if they are made of water, then it removes many possibilities, like powders, or the 99.9% of chemicals with different refractive indices to water.

    Secondly, the "true believers" in the conspiracy theory often do not accept even the simplest refutations of the basic claims of evidence. They will insist that contrails cannot persist for more than a minute, or that there are no gaps behind the engine on "chemtrail" planes, or that ordinary 22° halos indicate a different refractive index, or that contrails are persisting when it's too hot.

    As a debunker, the "true believer" is not your target audience. Since their argument is largely based on their conviction that they are correct, it's often impossible to even get them to consider that one or more of their claims of evidence might be wrong. With the true believer you will probably never get to the "absence of evidence" stage of the discussion. So it's important to recognize this. It's fine to discuss the claims of evidence with a true believer, especially in a public forum, as the dialog might be useful to those who have simply been taken in by the claims. But don't waste your time with the "absence" argument on people who think that contrails of any kind are physically impossible. The "absence" argument is only useful with people who can actually recognize that you have demonstrated an absence of evidence, and who don't yet realize that this absence of evidence is actually evidence of absence.

    Another example, 9/11 explosive demolition

    The 9/11 conspiracy theory organization "Architects and Engineers for 9/11 Truth" claims that the destruction of the towers was not caused by the planes' impact and subsequent fires, but was instead caused by powerful explosives placed on all the supporting columns on each floor of both 110 story towers and the 47 story WTC7 building.

    Now there's a number of obvious holes with this story, but we should ask what evidence would we expect of such an event? If you ask a truther, then they would tell you about videos of windows being blown out, or iron microspheres being found in the dust, or people hearing "explosions". These claims of evidence can be addressed in the usual way, by explaining how they would come about without explosives. But what expected evidence here is absent?

    One thing is any recorded audio of these explosions. If you compare video of a conventional demolition to video of the WTC tower collapses taken from similar distances, the former is punctuated by a series of astonishingly loud bangs, the latter is strangely silent, with just the roar of collapse. This absence of sound evidence does not rule out explosives, but it does rule out a large class of rapidly acting explosives, leaving only rather slow burning explosives, which would then conflict with other claims (like the rapid ejection of material being from explosions).

    A more significance absence is the absence of evidence of a detonation mechanism. Consider what AE911 are proposing - that literally thousands of quiet explosive charges have been set up in advance to go off in a manner that is so carefully timed as to be indistinguishable (to the worlds scientists) from progressive collapse. Traditional demolitions require miles of detonation (det) cord, with the timing of the charges being largely determined by the length of the cords. Det cords leave evidence. No evidence of det cords was found. So that's pretty strong evidence that det cord was not used.

    So we narrow down the theory a little, if not det cord, then what? Radio controlled detonators? 1000+ radio controlled detonators that all went off perfectly at the right time, none of them failed? Of course it's not impossible, in theory. But in practice you'd expect to find some evidence of this (if you even accept such a theory in the first place). Some detonators would probably fail, some evidence of the detonators would remain. But no evidence was found.

    And consider that the floors that failed first were the floors around the region of impact, this requires that the detonators and the explosives be incredibly hardened to resist impact and fire (which also rules out det cord even more). This hardening would greatly increase the likelihood that evidence of them would survive the collapse, and since the impact zone could not be precisely guaranteed, then a significant percentage of the detonators and explosives would have to be highly hardened, and hence more likely to leave some evidence in the debris pile.

    Notice what is happening here. The absence of evidence does not prove that there were no explosives, but it greatly restricts the possible ways the theory could have worked. In part this is reductio ad absurdum - a form of argument where you show that for a postulated fact (explosives bringing down the WTC towers) to be true, then logic requires the believer to accept essentially absurd requirements: Silent fireproof high explosives in explosion resistant containers with perfectly working vaporizing wireless armored detonators, all of which being unknown to the world at the time (or in the 15 years since).

    So in summary: once you have addressed the claims of evidence that are used to justify a theory, then the next step is to ask "what evidence would we expect, if this theory is correct". Then see what of this evidence is missing, and what implications that has for the validity of the theory. This will then narrow the forms the theory can taken, generally to the level of either absurdity (magic explosives) or banality (no explosives). While debunking is largely focussed on examining individual claims of evidence, at some point it's very useful to focus on evidence that has not been claimed - the evidence that is absent, and by that absence of evidence indicates evidence of absence.
    Last edited: Jul 5, 2016
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  3. MikeC

    MikeC Senior Member

    A basic (and silly) example is the picture in your post...I see a black swan.

    There are no elephants in the picture.

    does the absence of elephants mean there are no elephants in the picture, or just that I don't see them??
  4. Mick West

    Mick West Administrator Staff Member

    It's an indication that there are no elephants. However not a good example as any elephant would only be visible in indistinct reflection, so it's not strong evidence of absence.
  5. MikeC

    MikeC Senior Member

    don't force me to write that the swan has not been crushed by the elephant because the swan looks uncrushed!! ;)

    Evidence of absence on wiki
    Last edited: Jul 5, 2016
  6. deirdre

    deirdre Moderator Staff Member

    why do people have such a hard time comprehending the "no click policy"?

    That said you'd be better off proving there are no wild elephants around the Vacha Reservoir in Bulgaria. Of course, Micks pic is cropped and that could be an elephant on the shore. :p

  7. MikeC

    MikeC Senior Member

    I did say in the pic - not around the shore :p
  8. Trailspotter

    Trailspotter Senior Member

    Swans are neither white, nor black :p
  9. derwoodii

    derwoodii Senior Member

    i'm a little dizzy but i think I've grasped the concept or at least some of it better.
  10. mm1145

    mm1145 Member

    "absence of something where it is predicted is evidence"
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  11. tinkertailor

    tinkertailor Active Member

    Does anybody know what the context behind Sagan saying this was? I'm not the most knowledgeable person regarding him.
    It might be helpful for us debunking folks to counter the "but Sagan said it, so it totally can be said about chemtrails!" argument if we link to this thread and add "he said it in reference to ____, which was deemed likely to exist despite there having not been evidence. in recent years we have determined that he was right about ____, due to finding evidence with better technology" etc.
  12. NoParty

    NoParty Senior Member

    Yes, it comes from his essay The Fine Art of Baloney Detection

    1. In addition to teaching us what to do when evaluating a claim to knowledge, any good baloney detection kit must also teach us what not to do. It helps us recognize the most common and perilous fallacies of logic and rhetoric. Many good examples can be found in religion and politics, because their practitioners are so often obliged to justify two contradictory propositions. Among these fallacies are:
      • · ad hominem—Latin for “to the man,” attacking the arguer and not the argument (e.g., The Reverend Dr. Smith is a known Biblical fundamentalist, so her objections to evolution need not be taken seriously);
      • · argument from authority (e.g., President Richard Nixon should be re-elected because he has a secret plan to end the war in Southeast Asia—but because it was secret, there was no way for the electorate to evaluate it on its merits; the argument amounted to trusting him because he was President: a mistake, as it turned out);
        * A more cynical formulation by the Roman historian Polybius: Since the masses of the people are inconstant, full of unruly desires, passionate, and reckless of consequences, they must be filled with fears to keep them in order. The ancients did well, therefore, to invent gods, and the belief in punishment after death.
    • · argument from adverse consequences (e.g., A God meting out punishment and reward must exist, because if He didn’t, society would be much more lawless and dangerous—perhaps even ungovernable.* Or: The defendant in a widely publicized murder trial must be found guilty; otherwise, it will be an encouragement for other men to murder their wives);
    • · appeal to ignorance—the claim that whatever has not been proved false must be true, and vice versa (e.g.,

      There is no compelling evidence that UFOs are not visiting the Earth; therefore UFOs exist—and there is intelligent life elsewhere in the Universe. Or: There may be seventy kazillion other worlds, but not one is known to have the moral advancement of the Earth, so we’re still central to the Universe.)
      This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence.
    • · special pleading, often to rescue a proposition in deep rhetorical trouble (e.g., How can a merciful God condemn future generations to torment because, against orders, one woman induced one man to eat an apple? Special plead: you don’t understand the subtle Doctrine of Free Will. Or: How can there be an equally godlike Father, Son, and Holy Ghost in the same Person? Special plead: You don’t understand the Divine Mystery of the Trinity. Or: How could God permit the followers of Judaism, Christianity, and Islam—each in their own way enjoined to heroic measures of loving kindness and compassion—to have perpetrated so much cruelty for so long? Special plead: You don’t understand Free Will again. And anyway, God moves in mysterious ways.)
    • · begging the question, also called assuming the answer (e.g., We must institute the death penalty to discourage violent crime. But does the violent crime rate in fact fall when the death penalty is imposed? Or: The stock market fell yesterday because of a technical adjustment and profit-taking by investors—but is there any independent evidence for the causal role of “adjustment” and profit-taking; have we learned anything at all from this purported explanation?); [Another 5 or 6 standard fallacies round out his list]
    So the line merely appears in a list of well know informal fallacies: he's using it in the standard ad ignorantiam way...not attempting to say anything new...he's just basically presenting laypeople with a useful brochure of fallacious arguments to be avoided. In my opinion, many people are using the quote in a way he would dislike--believing that invoking his name lends credibility to their wild claims--with little regard for what he really meant. "Quote-mining," I think the kids call it these days.
    In short, as wonderful a brain as CS had, he certainly wasn't saying anything new in '95 about the well-worn 'appeal to ignorance' fallacy.

    The essay appears in the book The Demon-Haunted World...just a year or so before Sagen's death in '96.
    Last edited: Jul 6, 2016
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  13. deirdre

    deirdre Moderator Staff Member

    its in
    Demon-Haunted World: Science as a Candle in the Dark
    By Carl

    dont know if thats where he originally said it of course, he very well might have said it in an earlier talk or book as well.
  14. deirdre

    deirdre Moderator Staff Member

    he says it on this page too. But the Baloney chapter comes before this chapter.
  15. deirdre

    deirdre Moderator Staff Member

    perhaps a tad bit off topic , but i found this passage from his book meaningful as far as Ct thinking also. In regards to 'they have no right' to harass families or threaten pilots because of "absence of evidence is not evidence of absence" rationalizing.
    no right.JPG
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  16. tadaaa

    tadaaa Active Member

    I used to have a saying, when discussing these things with my older brother many many moons ago

    "Nothing is impossible, just increasing levels of implausibility"
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  17. AumuA

    AumuA New Member

    Wow, what a great post, Mick! You know, this is what we pay for here: quality content and clearly and entertainingly presented, rock solid logic. There's just one thing.. I can't seem to locate your Donate button. Where is it? Well, regardless I'm sure you must have a GoFundMe or an Indigogo I can contribute to, right? I mean, tell me, how can I support you in your crusade to bring truth to the sheeply masses?
    Last edited: Jul 7, 2016
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  18. Lisa P

    Lisa P Active Member

    Off topic but maybe the lack of funding is what makes this site so great, no vested interests. AumuA have you looked in the mirror lately? We are human beings not sheep but if you want to segregate people keep doing what you are doing.
  19. Landru

    Landru Moderator Staff Member

    There is no funding. Mick supports this himself. The other admins donate their time.
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  20. mrfintoil

    mrfintoil Active Member

    If you can't prove it, why assume it?

    A fair question that should be posed to those who insist on keeping believing despite lack of evidence.
  21. AumuA

    AumuA New Member

    Sorry, guess I didn't quite snark enough there. ;) Just to be 100% clear, my post was entirely tongue-in-cheek.

    Except for the 'great post Mick' part. I meant that.
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  22. Gary Cook

    Gary Cook Active Member

    I don't think chemtrails is a good example, personally.

    Nobody here has shown the increase, in what debunkers say is purely persistent contrails/clouds, is in correlation with the increase in air travel or not.

    I don't think people should claim that chemtrails exist for sure but I don't see an issue with them saying they believe there may be chemtrails because of the increase in the kind of trails and clouds that are seen.

    Just an example relating to the original post really. Not trying to debate chemtrails per se. To me, there is worse going on in plain view.
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  23. Mick West

    Mick West Administrator Staff Member

    But the OP is about absent evidence that the theory necessitates. So how exactly are the chemtrail examples wrong?

    Please read it again.
  24. Marin B

    Marin B Active Member

    That's what I thought. A tongue in-cheek emoticon could be useful. Though I'm not sure what one would look like.
  25. vooke

    vooke Active Member

    Excellent post.
    I think the important question is to determine when absence of evidence is actually evidence of absence. This appears to be when the presence is reasonably understood and as such we can make accurate predictions for what accompanies it. Absence of these predictions is thus taken as absence of evidence.

    But a believer can always challenge the reasonableness of your understanding. A 9/11 truther may for instance imagine existence of silent, zero-residue ordinances that are secretive. Deployment of such may defy all your predictions which are based on the presently and widely known characteristics of ordinances (loud, flashy,leaves residue....)

    This appears incredulous but I have witnessed it;
    There was a structure that collapsed inside a church compound killing 116 visitors in Nigeria. There being no earthquake, flooding and given the negligible likelihood of controlled demolition of a building already occupied 24/7 and under heavy security, a theory emerged that a super-secret 'infrasonic' weapon brought down the building.

    The diehard followers of the 'prophet' actively peddled this, and over a debate, I questioned the absurdity of the theory mainly the limits of modern science and millitary technology. The retort was, 'you don't even know half of everything'
    Last edited: Jul 11, 2016
  26. Hama Neggs

    Hama Neggs Senior Member

    Isn't the OP just describing the situation when the CT person tries to turn the tables and says you can't prove their theoretical "thing" DOESN'T exist? It's just them trying to insist that you prove the negative, which is always impossible.
  27. Mick West

    Mick West Administrator Staff Member

    No it's about the case when you CAN prove the negative, at least to a reasonable degree of certainty.

    You can prove a negative is the presence of something would create some observable evidence. For example, if I claimed there was a 400 pound bear living in my car, then it would be very easy to prove that this was not so by the absence of any evidence of this bear in my car.
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  28. Hama Neggs

    Hama Neggs Senior Member

    Yeah, but the chemtrailer's demand is more like proving there aren't any bears in any cars, anywhere.
  29. Mick West

    Mick West Administrator Staff Member

    And I always say you can't prove there's no spraying program that leaves no evidence. But this isn't about that. It's about when they do make claims that, if true, would leave some evidence.

    For example, if the theory is that chemtrails are made of powder, like aluminum oxide, then there would be evidence of trails that act like aluminum oxide, and not like ice crystals.

    In this case there is no evidence of anyone ever seeing such trails. This is is very strong evidence that there's no such spraying program, at least on any significant scale.

    It narrows down what the possible conspiracy theory can be.
  30. Hama Neggs

    Hama Neggs Senior Member

    The latest thing I have seen is the claim that they would act the same because the supposed aluminum dust is acting as condensation nuclei. They want you to prove that idea is NOT possible. There is just no end to the theory being adapted to "explain away" any and all objections. It's like when you say: Why would the pilots be willing to spray their own families?", the response is: 1) They're crazy 2) They don't know what they're spraying 3) They have an antidote 4) They are forced to do it. 5) Whatever... It's always a never-ending Whackamole. I'm starting to think it's not worth doing anymore. It's too frustrating.

    As far as a LACK of evidence, I get: "That's because it's a COVERT program!" Sheesh!
  31. deirdre

    deirdre Moderator Staff Member

    good. that means they are admitting what they see IS CONtrails. contrails acting like contrails.
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  32. Hama Neggs

    Hama Neggs Senior Member

    It's really just been one or two persons, over the last year or so. When confronted with the idea that it means they couldn't tell the difference, they stop engaging or say: "Then you can't prove it's NOT the aluminum dust".
  33. MikeC

    MikeC Senior Member

    If (supposed) aluminium dist acts exactly like ice crystals then that is because the (supposed) aluminium dust IS ice crystals and not actually aluminium dust.

    QED :)
  34. vooke

    vooke Active Member

    I don't know if this would help but if you are examining evidence for and against a particular theory and we get to such stalemate of inconclusive point(s), then you should suggest that they be backed up by other stronger evidence to justify clinging on to them. I mean for instance if it is a question of whether the that was water vapor or aluminum dust, ask them for further evidence that tips their conviction towards aluminum dust.
  35. Hama Neggs

    Hama Neggs Senior Member

    You may have misunderstood. They are claiming that it is aluminum dust which is serving as condensation nuclei for the trails we see. That would seem to indicate to me an invisibly small amount of aluminum, but whatever... None of their claims make sense when analyzed critically, but one grows weary of picking them apart only for them to alter them to another theory you have to debunk.
  36. Toni Misten

    Toni Misten New Member

    I have been to the engineers site, and I recall them stating that cutting charges, micro-thermite, was placed on key points of the buildings, but not explosive charges perse. But it's been a while so I may be off on that.
    Last edited by a moderator: Jul 29, 2016
  37. Mick West

    Mick West Administrator Staff Member

    But they also claim that huge girders were thrown hundreds of feet by (silent) explosions.