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Author Topic:   Absence of evidence IS evidence of absence
DNAunion
Inactive Member


Message 1 of 89 (64702)
11-06-2003 8:50 AM


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:

quote:
ABSENCE OF EVIDENCE IS EVIDENCE OF ABSENCE

Apparently, not everyone believes this statement to be correct.

quote:
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"?

quote:
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

****************************************
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.
****************************************


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]


Replies to this message:
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crashfrog
Inactive Member


Message 2 of 89 (64705)
11-06-2003 8:57 AM
Reply to: Message 1 by DNAunion
11-06-2003 8:50 AM


Apparently, not everyone believes this statement to be correct.

Yes. For instance, all logicians and scientists. Where have you been?

This absence of evidence is evidence of absence...what it is not is proof of absence.

Indeed. Therefore it's fallacious to try and base reasoning off the position that there's an absence of life as we don't know it, as you are trying to do when you suggest the universe is fine-tuned for life.


This message is a reply to:
 Message 1 by DNAunion, posted 11-06-2003 8:50 AM DNAunion has not yet responded

Replies to this message:
 Message 4 by mike the wiz, posted 11-06-2003 9:02 AM crashfrog has responded
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Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 3 of 89 (64706)
11-06-2003 8:58 AM
Reply to: Message 1 by DNAunion
11-06-2003 8:50 AM


Try this analogy: you want to find our if a football is on a football pitch. You check one blade of grass - there's no football on it.

Is this evidence for or against the football being on the pitch?

You're doing the same thing.


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mike the wiz
Member
Posts: 4637
From: u.k
Joined: 05-24-2003


Message 4 of 89 (64708)
11-06-2003 9:02 AM
Reply to: Message 2 by crashfrog
11-06-2003 8:57 AM


'Indeed. Therefore it's fallacious to try and base reasoning off the position that there's an absence of life as we don't know it,'

According to this logic there is no absence of God, according to Crashfrog then?

Sorry Crash, I couldn't resist it, I know you argue that there is no God because there is no evidence. But there is definately no evidence of other life .


This message is a reply to:
 Message 2 by crashfrog, posted 11-06-2003 8:57 AM crashfrog has responded

Replies to this message:
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crashfrog
Inactive Member


Message 5 of 89 (64709)
11-06-2003 9:07 AM
Reply to: Message 4 by mike the wiz
11-06-2003 9:02 AM


According to this logic there is no absence of God, according to Crashfrog then?

There's no evidence for an absence of all possible, concievable gods, no. There could very well be a god. There's just more than enough evidence that the god that you believe in doesn't exist.

You can use an absence of evidence as evidence of absence when you're pretty damn sure you'd know where the evidence would be, if it existed at all. In the case of life as we don't know it, since we don't know about it (by definition), we don't know where the evidence for it would be. Therefore claiming that there's an absence of evidence for it is premature, since he haven't looked in enough places yet.

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?

Sorry Crash, I couldn't resist it, I know you argue that there is no God because there is no evidence. But there is definately no evidence of other life .

Of course there's not. We haven't even looked where we would expect the evidence to be, yet. (In our defense, we haven't been able to yet.)

[This message has been edited by crashfrog, 11-06-2003]


This message is a reply to:
 Message 4 by mike the wiz, posted 11-06-2003 9:02 AM mike the wiz has responded

Replies to this message:
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Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 6 of 89 (64710)
11-06-2003 9:08 AM
Reply to: Message 4 by mike the wiz
11-06-2003 9:02 AM


Absence of evidence where there should be evidence IS evidence of absence.

God => Whatever .'. ~Whatever => ~God.

[This message has been edited by Mr Jack, 11-06-2003]


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mike the wiz
Member
Posts: 4637
From: u.k
Joined: 05-24-2003


Message 7 of 89 (64711)
11-06-2003 9:10 AM
Reply to: Message 5 by crashfrog
11-06-2003 9:07 AM


quote:
'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?'

Fair enough, good analogy. You kicked butt.

quote:
'There could very well be a god.'

Let's leave it here, or I'll have to change my name to 'topicdestroyer'.

[This message has been edited by mike the wiz, 11-06-2003]


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Replies to this message:
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NosyNed
Member
Posts: 8839
From: Canada
Joined: 04-04-2003
Member Rating: 2.7


Message 8 of 89 (64719)
11-06-2003 9:59 AM
Reply to: Message 2 by crashfrog
11-06-2003 8:57 AM


Crash, I think DNA is making a very good point. He has been clear that absence of evidence isn't a proof of anything untill ALL possible searches have been done.

A problem may be that you are trying to be black and white. For example, in the football analogy. No one would conclude there is no football after looking at one blade of grass. But after some number of random searches (short of the whole field) more and more of us will conclude there is no football. The line between "I don't know if the football is there" and "It ain't there" isn't sharpe. We would all drift over it with differing amounts of evidence and many of us would be in a middle zone for awhile.

I was once at a lecture of Freeman Dyson's. For some reason the issue of extraterristrials came up. I asked what his take on the subject was. He said something like 'Unfortunately I have to conclude they are not there'. Why? Where are they? Absense was as much evidence as he had and he was willing to draw a conclusion from that (tentively perhaps).


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Dan Carroll
Inactive Member


Message 9 of 89 (64721)
11-06-2003 10:10 AM
Reply to: Message 1 by DNAunion
11-06-2003 8:50 AM


The Jedis, no. The Second Foundation, yes.
This message is a reply to:
 Message 1 by DNAunion, posted 11-06-2003 8:50 AM DNAunion has not yet responded

  
Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 10 of 89 (64723)
11-06-2003 10:11 AM
Reply to: Message 8 by NosyNed
11-06-2003 9:59 AM


NosyNed,

The point is that a single blade of grass doesn't constitute evidence on it's own, it's only by gathering a body of evidence (thousands of grass blades) that you can consider it evidence for the football's presence or not. In the case of alien life, or ~LAWKI, we have just one example and gee-willy-wizz the life as we know is Life As We Know It. This is nowhere near evidence that life can only be LAWKI. It's like throwing a dice once, getting a number and concluding the dice is biased.

A single data point is not evidence for anything.


This message is a reply to:
 Message 8 by NosyNed, posted 11-06-2003 9:59 AM NosyNed has responded

Replies to this message:
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NosyNed
Member
Posts: 8839
From: Canada
Joined: 04-04-2003
Member Rating: 2.7


Message 11 of 89 (64727)
11-06-2003 10:34 AM
Reply to: Message 10 by Dr Jack
11-06-2003 10:11 AM


I think both sides of this agree to some degree. We do NOT have enough evidence to conclude anything firmly. They are both just coming down on their favourite side of the arguement. Some people just like to carry the arguement on further than it needs to be.

A single data point is not evidence for anything.

Yes it is! It isn't very much evidence but it is evidence. The chances of the football being there are just a shade smaller now. And if the football had been there that single data point would have been very strong evidence.
This message is a reply to:
 Message 10 by Dr Jack, posted 11-06-2003 10:11 AM Dr Jack has responded

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Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 12 of 89 (64728)
11-06-2003 10:40 AM
Reply to: Message 11 by NosyNed
11-06-2003 10:34 AM


True. I should have said 'You cannot generalise from a single data point.'
This message is a reply to:
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DNAunion
Inactive Member


Message 13 of 89 (64741)
11-06-2003 12:54 PM
Reply to: Message 1 by DNAunion
11-06-2003 8:50 AM


quote:
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 ...

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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crashfrog
Inactive Member


Message 14 of 89 (64742)
11-06-2003 12:58 PM
Reply to: Message 13 by DNAunion
11-06-2003 12:54 PM


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.

Now who's misrepresenting arguments? Had you read closer you would know that in fact, what we were saying is "since you cannot prove that 'life as we don't know it' doesn't exist, you can't make a claim that it doesn't exist, which is implicit in a claim of 'fine-tuning'."

This is a substantial point that you have yet to rebut.


This message is a reply to:
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Replies to this message:
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DNAunion
Inactive Member


Message 15 of 89 (64744)
11-06-2003 1:10 PM
Reply to: Message 7 by mike the wiz
11-06-2003 9:10 AM


quote:
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?

quote:
Fair enough, good analogy. You kicked butt.

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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