Absence of Evidence is not Evidence of Absence

Absence of Evidence is not Evidence of Absence ... or is it?

“Absence of evidence is not evidence of absence”, is a saying frequently quoted by cryptid proponents, more especially Bigfoot proponents, in an attempt to dismiss the fact that no type specimens have ever been found. Ironically, most of those who employ this saying do not realize that they are quoting a skeptic that did not believe in Bigfoot, or alien animals, or prehistoric survivors, or any of the other popular figures of cryptozoology.

The quote can be found on page 213 of Carl Sagan’s, “The Demon-Haunted World—Science as a Candle in the Dark”, and is strictly used to criticize “argumentum ad ignorantiam (“appeal to ignorance”), which is to say that “something must be true because it cannot be proven false, and vice versa”.

I do not entirely agree with Carl Sagan’s quote. In my opinion, absence of evidence can only be evidence of absence. Missing evidence is, after all, a very clear indication that the evidence is probably not there. However, I realize that absence of evidence is not proof of absence. But if Dr. Sagan had stated it that way, then we would not have a nifty little, easily memorized antimetabole that, despite my disagreement, conveys an apparent truth in the proper context.

Contrary to many proponents, I believe that cryptozoology falls short of the proper context, and I believe that Dr. Sagan agrees. On page 171 of the above quoted book, Dr. Sagan introduces us to the invisible dragon in his garage; a dragon that is impervious to the five senses. After introducing us to his undetectable dragon, he states, “What I’m asking you to do comes down to believing, in the absence of evidence, on my say-so”, which is to say, believing the undetectable dragon is evident, despite the absence of evidence. The point Dr. Sagan is making is that we should not accept “say-so” without the support of empirical evidence. In other words, in this case, the “absence of evidence” is indeed “the evidence of absence”.

Bigfoot and the other popular cryptids largely fit the criteria of Dr. Sagan’s undetectable garage dragon. After using his invisible dragon to show us the inefficacy and unreliability of “say-so”, Dr. Sagan writes, “The only thing you’ve really learned from my insistence that there’s a dragon in my garage is that something funny is going on inside my head.”

Granted, a garage is a tad smaller and easier to go through than thousands of square miles of forest and woodland. But I must nevertheless ask; is something funny going on inside someone’s head when they expect everyone to unquestionably believe that the existence of Bigfoot, or any other unlikely cryptid, is absolute; especially when it appears to be just as invisible and undetectable as Dr. Sagan’s hypothetical garage dragon?

Sadly, Carl Sagan is no longer with us to clarify his quote, but it is my opinion that Dr. Sagan logically believed that absence of evidence can indeed be evidence of absence.

Feel free to to comment.

COMMENTS

Dr. Sagan did clarify his quote, I don't remember where but I'll try to find the reference, and his clarification fits your definition to a "T".

Sagan, in fact, was not the first to use that statement (and he says so). I'll try to find that reference too. It's in one of his books; I don't remember which. I have most of them on my shelf, as he is my favorite author of nonfiction and speculation.

I agree, the statement is used far too often in cryptozoological arguments, in a way in which Dr. Sagan did not intend.

-kittenz

Thank you, kittenz. It would be great if you could locate Dr. Sagan's elaboration of the quote.

Karl

The full quote from 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."

It is very obtuse to suggest that Carl Sagan is endorsing that position he is calling an argument from ignorance. It is equally obvious that the aphorism is not Sagan's, but one he is explicitly criticizing.

And well he should criticize it, because you are correct, absence of evidence *can only be* evidence of absence.

Let H be a hypothesis that something is true, and E be an observation of evidence supporting H. The probability of H given E is P(H|E), and the probability of H given not-E is P(H|~E).

E being evidence of H means that P(H|E) > P(H). If P(H|E) > P(H) then conversely by Bayes' Theorem we can prove that P(H|~E) < P(H). So, it is mathematically provable: absence of evidence is *always* evidence of absence.

Robin Lionheart

Thank you for pointing out the obvious criticism of the phrase. I don't know how I couldn't see that after reading it so many times.

I had to google your equation to better understand it, and found that you've been busy. You're a scary individual Robin; I'm just glad I don't have to debate against you.

Karl

Bayes Theorem is used to indicate the probability of an event. If I have a box without any evidence in it then of course the probability of reaching my hand in the box and pulling out evidence is 0. However, this does not mean that evidence does not exist. Nor does it mean that lack of evidence proves non-existence. All it means is that the evidence has not yet been presented or found. Before the use of telescopes there was no evidence of other galaxies. That was not evidence that there were no other galaxies - it just meant that the evidence was not yet discovered.

Jared Ricks

Well, hey there cousin; first you tell me I'm wasting my time with this stuff, then you feel compelled to confront an argument. Sometimes you've just got to speak out, eh?

Let's say you're looking for tangible evidence of a creature in the box; how many times would you have to look in the empty box before you determined that the evidence simply wasn't there?

In the case of galaxies; (before the telescope), that box was simply closed, making it extremely difficult to determine if the evidence was there or not, the telescope simply "opened" the box and removed all doubt.

Karl

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