Absence of evidence IS evidence of absence

Do aliens exist? Does SuperMan? What about the Saiyan race, or the Jedi Knights, or the fuzzy little Gremlins that transform into nasty beasts when they get wet?In the absence of any supporting empirical evidence, something (such as the beings listed above) is mere unsupported speculation.A related concept goes like this:

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ABSENCE OF EVIDENCE IS EVIDENCE OF ABSENCE

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Apparently, not everyone believes this statement to be correct.

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Just out of curiousity... when a person argues using one of the most fundamental of all known logical fallicies, is it best to simply not respond, or to try and reason with the aforementioned "brick wall"?

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I just wanted to make sure that the proper phrase is made available so I pulled this from a webpage covering Carl Sagan's Baloney Detection Kit

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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.
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This relates back to an earlier statement I made in another thread. But when I made my statement ABSENCE OF EVIDENCE IS EVIDENCE OF ABSENCE I was well aware of Carl Sagan’s position on absence of evidence — it’s actually quite widespread in the SETI/astrobiology world (afterall, they do have an agenda in propagating their fallacy). However, there are several problems with trying to use that saying (and that’s all it is) against my statement.(1) I stated multiple times (in that other thread) that the lack of evidence does NOT prove nonexistence. Had I actually said it DID prove nonexistence, then I would have been guilty. But as things stand here in the real world, I made no logical fallacy.(2) The saying Absence of evidence is not evidence of absence is false; it is a fallacy. It is true only if it is modified, such as by equating the second occurrence of the word evidence with the word proof, as is implied by Sagan’s other statements. That is, it is true that Absence of evidence is not PROOF of absence, but that materially changes the saying to something else: evidence and proof are not the same thing. As literally stated - absence of evidence is not evidence of absence — is wrong, even if spoken by Sagan.In case someone still doesn’t get my (2), let me make a point I already made much earlier in the other thread:To be evidence, an observation does not have to point to the correct position: evidence can point to the wrong conclusion.In fact, science has used evidence that points to the wrong conclusion innumerable times in the past. Evidence gathered through some observations point to a certain conclusion and that conclusion is accepted (for example, when people stood still they didn’t feel the Earth moving — the absence of observed motion of the Earth was evidence that the Earth did not move: hence, the Earth was considered to be stationary). Later, when more and better evidence comes in that contradicts the conclusion the older evidence supported, a new conclusion is reached. But the older observations were still evidence for that other position. And this isn’t restricted to the old days. Even today, science is awash with opposing evidences. Two opposing sides pile up as much evidence as they can to support their position — but only one of the two positions can be correct. So it must be that evidence can point to a wrong conclusion. And consequently, observations that support a particular position are evidence for that position, even if that position turns out to be wrong.Anyone using **critical thinking** would have realized all of the above.Taking that all into consideration, let’s look at the question of whether or not life-not-as-we-know-it exists. Do we have any empirical observations whatsoever to back up the position that life-not-as-we-know-it exists? No. No signs of any kind of life based on anything other than normal biochemistry (organic compounds such as DNA, RNA, proteins, etc.) have been found here on Earth, nor in any meteorites recovered to date, nor in any Moon rocks retrieved from the Moon, nor in any tests performed on Mars’s surface; and SETI hasn’t even detected any radio signals from any kind of ETI (despite 30 or so years trying). This absence of evidence is evidence of absence...what it is not is proof of absence."Life as we don't know it" is unsupported speculation.[This message has been edited by DNAunion, 11-06-2003]

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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 ...

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This reasoning is close to what the side opposing me was arguing: since I cannot prove that "life not as we know it" doesn't exist, it exists.[This message has been edited by DNAunion, 11-06-2003]

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If you think you dropped your contact lens in the alleyway behind the bar, and you look out in front of the alley where the light is better and you can't find it, is that evidence that your contact lens is gone forever? Hardly. You haven't even looked where we would expect the evidence of your contact lens to be yet, so how can you claim there's no evidence?

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Fair enough, good analogy. You kicked butt.

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The fact is, not finding the contact lens in front of the bar IS evidence for its not being in front of the bar. Absence of evidence is evidence of absence.Trying to relate this to the "alien thingy", let's not forget that SETI has been "searching space" for decades without success: so in the analogy you'd at least need to have a friend who's been lookin behind the bar for some time and has also failed to find the contact lens. (Then both of your failures combined would be evidence that it is no longer present...perhaps it fell into trash and was taken away, or was stepped on and broken and the pieces scattered, or who knows what: or perhaps it is still there...doesn't matter).But the analogy doesn't work very well for the "life as we don't know it" debate anyway because we know for sure that the thing of interest (contact lens) exists/existed, we just can't find it at this moment. That doesn't parallel the situation in the original discussion.[This message has been edited by DNAunion, 11-06-2003]

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...what we were saying is "since you cannot prove that 'life as we don't know it' doesn't exist, ..

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It is silly to require someone to prove nonexistence in the first place! The rule is, science can never prove non-existence (at least not without an absolute full examination of the entire Universe).By your reasoning, science cannot claim there is no such thing as Superman. By your reasoning, science cannot claim there is no such thing as flying, fire-breathing dragons, or flying pink unicorns, or.......[This message has been edited by DNAunion, 11-06-2003]

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I would have absolutely no problem if they argued "There's not enough data gathered yet, so we can't come to a conclusion yet".

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Actually, that's basically what I said...multiple times (it hinges on what you mean by a conclusion, which could be definitive or tentative).I said we don't know which position is correct, and I said that it is an assumption that life as we know is the only kind of life that can exist, and so on.

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However, he's arguing that there's enough data.

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Uhm...no I am not. Geez dude, read my posts sometimes before claiming to know what I say.[This message has been edited by DNAunion, 11-06-2003][This message has been edited by DNAunion, 11-06-2003]

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I would have absolutely no problem if they argued "There's not enough data gathered yet, so we can't come to a conclusion yet". However, he's arguing that there's enough data.

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Uhm...no I am not.

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Then what is your argument that this universe is fine-tuned based on?

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I’ll let you read it yourself. I stated this in the other thread.

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Based on the supported (but not proven) assumption that life is restricted to LAWKI, the Universe is fine-tuned for life. For example, make the strong force a few percent stronger or weaker and the Universe is no longer compatible with life. Same goes for similar changes in the strengths of the weak force, the gravitational force, or the electromagnetic force. Even though the four forces range over approximately 40 orders of magnitude, a small (< 5%?) change in any one of them means no life.

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I will also point out that this thread has left its original topic.

I did a little search of the internet to see what others have said. Most hits for "absence of evidence is evidence of absence" seemed to be from general lay people discussing philosophy/religion. I ignored those. I saw several where the phrase was abused by those rejecting it, with the naysayers explicitly or implicitly equating it with "absence of evidence is proof of absence". And I did find a few that confirmed what I said.

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Absence of Evidence IS Evidence of Absence!

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Fritz A. Seiler, Ph.D.
Principal
Sigma Five Associates (Page Not Found | Vanderbilt University)

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Indeed, any thinking scholar would readily admit that an absence of evidence is evidence of absence. It is not proof of absence - but it certainly amounts to evidence. (http://www.nunki.net/PerRenput/Reaction/ReplyKitchen2.html)

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Another favorite fallacy is the Argument from Ignorance, or the implication that if something is not disproved then it must be true. "There is no proof that alien abductions have not occurred; therefore at least some of the abductees must be telling the truth." But there is no compelling evidence that even one person has been abducted by aliens, nor is there hard evidence even that extraterrestrial beings are visiting Earth, so it’s rather premature to say that abductees’ claims ought to be believed. In some cases, absence of evidence is evidence of absence, particularly when said evidence must exist for the claim to be believed. Abductees, to a man, suffer from an acute absence of evidence. scientium.com

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Contrary to what has been said at a previous SEG meeting, absence of evidence is evidence of absence--it is not proof of absence; but it is very definitely evidence. www.sysconn.com

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edited urls to fix page width - The Queen[This message has been edited by AdminAsgara, 11-06-2003]

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Absence of evidence for your hypothesis does not mean that the opposite of the hypothesis is true, nor does it mean that the hypothesis must be false. Likewise, absence of evidence against your hypothesis does not mean that the hypothesis is true, or that the opposite of your hypothesis is false. This seems clear enough, but it happens often that people make mistakes this way.
http://www.sunspot.noao.edu/sunspot/pr/science-11.html

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This one doesn't count against me (if anything, it supports exactly what I have said).This quote incorrectly equates evidence with "proof": i.e., whether a position is actually true or false. This is what I mentioned before: many of the naysayers invoked this flawed distortion of the actual statement, so they are knocking over a strawman.--------------------------------------------------------------------------------

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Professor Terje Traavik, the Scientific Director of the Norwegian Institute of Gene Ecology, Norway, will present Risks of genetic engineering: absence of evidence is not evidence of absence, in C1 at 7pm.
http://www.newsroom.canterbury.ac.nz/stories/03100801.htm

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This one doesn't count either...no context that supports your position over mine. For all we know Terje also distorts the actual statement (as the quote you gave right above it did).

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Absence of evidence is not evidence of absence is something every investigator should know, but too few do. Along with Interaction 2: compare effect sizes not P values, these articles describe two of the most common fatal mistakes in manuscripts submitted to research journals. The faulty reasoning leading to these errors is so seductive that papers containing these errors sometimes slip through the reviewing process and misinterpretations of data are published as fact. ... Page Not Found | Tufts University

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Nor does this one count against me as it too is looking at what is and is not FACT. This again implicitly changes the statement being rejected to "absence of evidence is proof of absence", which is not the actual statement of interest.[This message has been edited by DNAunion, 11-06-2003]

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... If Giulivi means that there are no reported cases of CJD or vCJD that have been transmitted through the blood supply, then he is right. But this lack of evidence does not mean that such transmission is not occurring. Giulivi is making an all-too- common error, since "the absence of evidence is not evidence of absence.
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Disclaimer - Electronic Collection

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Ditto - doesn't count against me. This author is also incorrectly swapping out the actual word evidence for a stronger meaning, such as proof. Of course lack of observations to date of its being transmitted by that mode does not PROVE that it can't be, but the lack of observations to date of its being transmitted by that mode is EVIDENCE that it can't be.[This message has been edited by DNAunion, 11-06-2003]

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"Absence of evidence is not evidence of absence"
- Carl Sagan, "Dragons of Eden"

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No good either. We have no idea what context is involved. Is Sagan, like a lot of the others you quoted, changing the statement by substituting a stronger word - such as proof - for evidence? Is he basically arguing that absence of evidence is not proof of absence?By the way, in case you didn't know this, there's another famous saying associated with Sagan: "Extraordinary claims require extraordinary proof". A claim that life based on silicon exists seems extraordinary to me...so do you have any extraordinary proof? Nah, you don't have any evidence whatsoever.PS: Note that this "Sagan saying" often times appears as "Extraordinary claims require extraordinary evidence". For some nutty reason, some people seem to think that the words PROOF and EVIDENCE mean the same thing, when clearly they don't.[This message has been edited by DNAunion, 11-06-2003]

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(concerning life explicitly):
'... or on any other planets, but our capability to collect such evidence is still poorly developed; thus "absence of evidence is not evidence of absence." '
http://nai.arc.nasa.gov/astrobio/astrobio_questions.cfm?q...

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First, not enough context is given to know if they too are manipulating the statement of interest. And when I tried to determine the context, the link you gave did not work (perhaps it will be working later tonight).Second, this is an ASTROBIOLOGY page - those people have an agenda...they have a need to continue to receive funds and lack of any evidence whatsoever is a bit discouraging to those shelling out big bucks. Of course they have a motive for jumping on the "absence of evidence isn't evidence of absence" bandwagon: it's a matter of job survival for them.[This message has been edited by DNAunion, 11-06-2003]

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In fact, logic has a name for this kind of shenanigan, an argument from ignorance; it hasn't been proven to exist, therefore it doesn't.

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Exactly WRONG. You have materially changed the position: what you knocked down is a mere strawman version of the actual saying "absence of evidence is evidence of absence".I've even pointed out this exact problem (proof or another similar much stronger term being substituted for mere evidence) multiple times in this one thread, and made it clear both in this thread and in the other thread that absence of evidence is NOT PROOF of absence.[This message has been edited by DNAunion, 11-08-2003]

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That I cannot find my car keys on my kitchen table is evidence that they are absent from the kitchen table. It isn't evidence (trust me!) that they aren't in the kitchen, they're probably on the microwave. The absence only becomes meaningful when the whole picture is being looked at.

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I disagree. Looking for them in the kitchen - on the table - and not finding them there is evidence that they are not in the kitchen. It isn't proof, and the evidence may even point towards the wrong conclusion (they very well may be on the microwave), but it is one piece of evidence that supports the position that the keys are not in the kitchen.Also, dragging this back to aliens (I know, this thread is not limited to aliens...), the analogy is weak as it involves searching for something we already know exists. If scientists search Loch Ness for a "monster" and don't find one, is that evidence of absence? And what about flying, pink unicorns? No ones ever seen one - is that evidence of absence?***************************************

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Indeed, any thinking scholar would readily admit that an absence of evidence is evidence of absence. It is not proof of absence - but it certainly amounts to evidence. (http://www.nunki.net/PerRenput/Reaction/ReplyKitchen2.html)

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[This message has been edited by DNAunion, 11-08-2003]