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Author | Topic: Absence of evidence IS evidence of absence | |||||||||||||||||||||||||||
DNAunion Inactive Member |
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: Apparently, not everyone believes this statement to be correct.
quote: quote: 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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crashfrog Member (Idle past 1498 days) Posts: 19762 From: Silver Spring, MD Joined: |
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.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 9.2 |
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: 4755 From: u.k Joined: |
'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 .
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crashfrog Member (Idle past 1498 days) Posts: 19762 From: Silver Spring, MD Joined: |
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]
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 9.2 |
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: 4755 From: u.k Joined: |
quote: Fair enough, good analogy. You kicked butt.
quote: 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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NosyNed Member Posts: 9004 From: Canada Joined: |
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 |
The Jedis, no. The Second Foundation, yes.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 9.2 |
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.
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NosyNed Member Posts: 9004 From: Canada Joined: |
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.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 9.2 |
True. I should have said 'You cannot generalise from a single data point.'
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DNAunion Inactive Member |
quote: 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 Member (Idle past 1498 days) Posts: 19762 From: Silver Spring, MD Joined: |
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.
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DNAunion Inactive Member |
quote: quote: 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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