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.
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.
In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence as positive proof of its non-occurrence.
— Copi, Introduction to Logic (1953), p. 95
If someone were to assert that there is an elephant on the quad, then the failure to observe an elephant there would be good reason to think that there is no elephant there. But if someone were to assert that there is a flea on the quad, then one's failure to observe it there would not constitute good evidence that there is no flea on the quad. The salient difference between these two cases is that in the one, but not the other, we should expect to see some evidence of the entity if in fact it existed.
— J.P. Moreland and W.L. Craig, Philosophical Foundations for a Christian Worldview (2003)
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.