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As it goes, absence of evidence is not evidence of absence. I have never in my life seen a mountain gorilla. I am aware of evidence that shows that they exist,

by dleslie 10y ago

As it goes, absence of evidence is not evidence of absence. I have never in my life seen a mountain gorilla. I am aware of evidence that shows that they exist, but at my age my grand parents did not have access to that same evidence; does that mean that mountain gorillas came into existence when the evidence was found? Almost certainly not, heh, but they would have been valid in disbelieving in the existence of such beasts. There could be a person behind me with a knife, poised to slay me; but I am fairly confident that this is not the case due to a number of factors. On a population scale, such incidents are vanishingly few, and I took precautions earlier today to ensure that my home was locked and secured; but that does not guarantee certainty of my safety. It would be a fallacy, perhaps post hoc ergo propter hoc, for me to believe in the certainty that there is no such individual behind me. Until I turn around and observe the empty space, I cannot lay claim to the existence or non-existence of that individual. We cannot observe all of Earth all of the time, and we cannot do so for the Solar system, or lay claim to having done so throughout history. This does not mean that aliens have visited us, or that they even exist; it means that we cannot be certain that they don't exist and that they haven't visited.

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mikeash 10y ago
That saying is nonsense. Absence of evidence is evidence of absence, with a strength that's determined by how likely it is that you should find evidence given what you've looked for. Taking your mountain gorilla example, do you have any in your house? I'm guessing you'll say "no." Well, how do you know that? Absence of evidence is not evidence of absence, after all. Yet if you look all over your house and don't see any, I'd guess you would accept that as sufficient reason to say that there aren't any in your house. But how can you say that if it's not evidence of absence? Now, finding no mountain gorillas in your house is very poor evidence for the absence of mountain gorillas on Earth, because you'd expect the same results whether or not they existed on Earth in general. On the other hand, if you've performed a thorough survey of their actual supposed habitat and didn't find any (maybe because they were driven to extinction) then you'd have pretty good (but not perfect!) evidence that they aren't there anymore. There's essentially no evidence for the absence of aliens in the universe. You'll see this reflected in the opinion of the experts; many, perhaps most, say that it's likely aliens are out there somewhere. There's decent evidence for the absence of aliens around Earth, since we see nothing and we'd expect to see something, but it's possible they're undetectable. There's great evidence for the absence of classical UFOs, since they're clearly things we'd expect to see if they were real (people keep saying they saw them!) but out of all these sightings, none of them have turned out to be anything remarkable at all.
- ewzimm 10y ago
  Most of what you're saying is right, but the saying isn't nonsense. It's just describing the problem that comes up when you can't find evidence of something. If its presence would necessarily present evidence, as with the gorilla (unless it's a mutant ninja gorilla), then it's evidence of absence. If its presence would not necessarily present any detectable evidence, then it's absence of evidence, meaning that you haven't really learned anything. With something like aliens, we don't actually know what evidence to look for, so as you're saying, there's no evidence of absence yet, only absence of evidence.
  - mikeash 10y ago
    It is nonsense, because the saying is absolute. It doesn't say "absence of evidence is not necessarily evidence of absence." In reality, evidence of absence sometimes is and sometimes isn't evidence of absence, it depends on the specifics. The fact that the saying is correct for some cases doesn't make it sensible, any more than saying "birds don't fly" is sensible because of penguins and ostriches.
    - ewzimm 10y ago
      It's just a cute aphorism that describes one of the main challenges in science. The "is not" is meant to convey that there are two distinct conclusions that we can make from seeing a lack of something and that it's important not to make the wrong choice while evaluating the situation. We can't automatically say "absence of evidence" or "evidence of absence" as it suits our preconceptions but need to look deeper into why something is missing.
      - mikeash 10y ago
        Well, every single time I've seen someone use that phrase, it hasn't been to point out that they're not equivalent, it's been to argue for the potential existence of something where there's zero evidence for it and lots of searching you'd expect to turn up evidence. Case in point here, we are not discussing aliens in general, where that phrase might be sensible, but rather aliens frequently visiting Earth over a period of years in the form of UFOs.

by dleslie 10y ago

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mikeash 10y ago

That saying is nonsense. Absence of evidence is evidence of absence, with a strength that's determined by how likely it is that you should find evidence given what you've looked for. Taking your mountain gorilla example, do you have any in your house? I'm guessing you'll say "no." Well, how do you know that? Absence of evidence is not evidence of absence, after all. Yet if you look all over your house and don't see any, I'd guess you would accept that as sufficient reason to say that there aren't any in your house. But how can you say that if it's not evidence of absence? Now, finding no mountain gorillas in your house is very poor evidence for the absence of mountain gorillas on Earth, because you'd expect the same results whether or not they existed on Earth in general. On the other hand, if you've performed a thorough survey of their actual supposed habitat and didn't find any (maybe because they were driven to extinction) then you'd have pretty good (but not perfect!) evidence that they aren't there anymore. There's essentially no evidence for the absence of aliens in the universe. You'll see this reflected in the opinion of the experts; many, perhaps most, say that it's likely aliens are out there somewhere. There's decent evidence for the absence of aliens around Earth, since we see nothing and we'd expect to see something, but it's possible they're undetectable. There's great evidence for the absence of classical UFOs, since they're clearly things we'd expect to see if they were real (people keep saying they saw them!) but out of all these sightings, none of them have turned out to be anything remarkable at all.

ewzimm 10y ago
Most of what you're saying is right, but the saying isn't nonsense. It's just describing the problem that comes up when you can't find evidence of something. If its presence would necessarily present evidence, as with the gorilla (unless it's a mutant ninja gorilla), then it's evidence of absence. If its presence would not necessarily present any detectable evidence, then it's absence of evidence, meaning that you haven't really learned anything. With something like aliens, we don't actually know what evidence to look for, so as you're saying, there's no evidence of absence yet, only absence of evidence.
- mikeash 10y ago
  It is nonsense, because the saying is absolute. It doesn't say "absence of evidence is not necessarily evidence of absence." In reality, evidence of absence sometimes is and sometimes isn't evidence of absence, it depends on the specifics. The fact that the saying is correct for some cases doesn't make it sensible, any more than saying "birds don't fly" is sensible because of penguins and ostriches.
  - ewzimm 10y ago
    It's just a cute aphorism that describes one of the main challenges in science. The "is not" is meant to convey that there are two distinct conclusions that we can make from seeing a lack of something and that it's important not to make the wrong choice while evaluating the situation. We can't automatically say "absence of evidence" or "evidence of absence" as it suits our preconceptions but need to look deeper into why something is missing.
    - mikeash 10y ago
      Well, every single time I've seen someone use that phrase, it hasn't been to point out that they're not equivalent, it's been to argue for the potential existence of something where there's zero evidence for it and lots of searching you'd expect to turn up evidence. Case in point here, we are not discussing aliens in general, where that phrase might be sensible, but rather aliens frequently visiting Earth over a period of years in the form of UFOs.