Thread: Absence of Evidence is Evidence of Absence

How?

So, based on your reply to the changed ring tone problem (and I'm not used to reading through such technical speak, especially right before bedtime, so forgive me if I'm wrong) an unsaid piece to your interpretation of this theory is that evidence of abscence is subjective to the observer. For example, if we took two people, gave person A knowledge of the alphabet from A-M and gave person B knowledge of the letters N-Z and had them stare at a wall with an O drawn on it, person A would rightfully say there is evidence of abscence of the alphabet from the wall whereas person B would (also correctly) say there is none? Because person A didn't have a knowledge base encompassing the part of the alphabet including O he would see evidence of abscence when in reality there is proof of existance of the alphabet?

Would I be correct in making that assumption, that according to what you've been saying evidence of abscence is evidence only to the observer and people with applicable knowledge base similar to that of the observer?

I'm not saying that evidence of ghosts in unreliable, or that the uncertainty is necessarily greater in this case. If one were to see a ghost as you describe that would be a matter of positive evidence. What I am saying is that from the personal perspective of someone who has never seen a ghost, or from the broader scientific perspective that there has never been documented evidence of a ghost, one cannot legitimately claim that evidence for ghosts is indeed absent, since we cannot be sure what that evidence would even look like if it were present.

I'm not saying that the existence of ghosts is untouchable either, only that barring the perspective of someone who believes they have in fact seen a ghost, this phenomena has not yet been touched at all.

I don't know, I think we've reached the point of a run around. I'm not even really questioning the fact that one can gleen certain probabilities from this exercise, only the deeper philosophical efficacy of the whole matter.

Originally Posted by rose_red

Would I be correct in making that assumption, that according to what you've been saying evidence of abscence is evidence only to the observer and people with applicable knowledge base similar to that of the observer?

Yes, that is exactly correct. Although I should point out that I haven't left this point "unsaid" at all; it's been a major topic of discussion in many of the posts posts following the OP. But given the sheer length of many of the posts in this thread (particularly my own), you can be forgiven for not having read them all.

Originally Posted by Xaqaria

I don't know, I think we've reached the point of a run around. I'm not even really questioning the fact that one can gleen certain probabilities from this exercise, only the deeper philosophical efficacy of the whole matter.

Fair enough.

Dub, is the following statement true or false? Absence of proof is not proof of absence.

Given that we are talking about inductive reasoning, it is clearly true. In logic, inductive conclusions are rendered more probable if they follow from the premises, but they are never logically entailed by those premises. Proof need not apply. In mathematics as well, failure to provide a proof is not a proof to the contrary; you have to actually prove the contrary (a refutation). Interestingly, even proving that a certain property is unprovable is not the same as refuting that property--hence the apparent paradox of the incompleteness theorems.

Very well written thread.

Originally Posted by DuB

Given that we are talking about inductive reasoning, it is flatly false. In logic, inductive conclusions are rendered more probable if they follow from the premises, but are not logically entailed by those premises. Proof need not apply. In mathematics as well, failure to provide a proof is not a proof to the contrary; you have to actually prove the contrary (a refutation). Interestingly, even proving that a certain property is unprovable is not the same as refuting that property--hence the apparent paradox of the incompleteness theorems.

You seem to agree with the statement. Are you sure you noticed the "not" in it?

Originally Posted by Universal Mind

You seem to agree with the statement. Are you sure you noticed the "not" in it?

Fixed...

Okay, we're on the same page then. So while you agree with my statement concerning proof, you disagree with the statement when the word "proof" is replaced with the word "evidence" because evidence only has to serve the probability of the truth of a negative claim while proof has to show it to be definately true. I agree with you. Absence of evidence of x does serve the probability of not x. So apparently the saying should involve the word "proof" instead of "evidence".

Originally Posted by Universal Mind

Okay, we're on the same page then. So while you agree with my statement concerning proof, you disagree with the statement when the word "proof" is replaced with the word "evidence" because evidence only has to serve the probability of the truth of a negative claim while proof has to show it to be definately true. I agree with you. Absence of evidence of x does serve the probability of not x.

Exactly.

Originally Posted by Universal Mind

So apparently the saying should involve the word "proof" instead of "evidence".

Well, it's not exactly that it should refer to proof rather than evidence, it's that people need to be clear about which version of the claim they're really asserting. I call the version involving "proof" the weak version (because it is obviously true, but neither interesting nor relevant) and the version involving "evidence" the strong version (because it is both interesting and relevant, but flat wrong). They are both bad arguments, but for very different reasons.

Unfortunately, people very often tend to get the two versions mixed up, typically such that they speak explicitly of the strong version but interpret it implicitly as the weak version (i.e., they speak of "evidence of absence" but really are talking about "proof of absence"). This gives the statement the illusion of being interesting, relevant, and true all at once. It isn't.

I think too many people really do believe that absence of proof is proof of absence. To them, the nature of the principle is not so obvious. The claim that not finding WMD stockpiles in Iraq proves that the WMD stockpiles were never there is a classic example, and people more commonly use the idea in regard to God's existence and his nonexistence. Of course, people also use reasonable theological arguments that sound a lot like the statements that miss the nature of proof but are different.

I certainly believe Abscence of evidence is NOT evidence of absence.

Allthough indeed alot of commercial mystics, doctors and healers( read hustlers) abuse this line to provide some credibility for their fraudulent businesses.

There are, however, many humanly experienced phenomena that science cannot explain. Not even remotely. Scientists are often so bold to then conclude this is evidence of falsehood/absence. However that seems rather bold and self absorbed to me. Instead of simply rediculing theories and phenomena, why do so many scientists never question their own methods of investigation?

You could off course study hallucinatory drug induced and psychotic/schizophrenic visions and the hearing of voices and, after having found no conclusive evidence of ther true objective existance, pass it off as false/illusionairy.

But why not wonder wether the instruments and the method of investigation you used were adequate and suitable for the "realm" of investigation?

Far too many scientists never doubt their own methods or instruments and are only analytical towards the subjkects of investigation. That is off course blind science.

When we keep this in mind, the line "Abscence of evidence is not evidence of absence" indeed holds much truth.

How many of you people here believe in Darwin's evolution theory? I for one do. We all know there are MANY missing links; absence of evidence, but I think it's safe to say that we have enough evidence to at least conclude that Evolution is a very credible theory.

We m,ay have many missing links, but the evidence of quite obvious links are out there: Fish > Frogs> Salamanders> Lizzards.... seems to reveal the truth of the Evolution theory. Even though there is still many conclusive evidence missing.

I think this is very much the same for any other subject that blind science seeks to declare as false. We have many pieces of evidence that suggest the existance of Telepathy, foresight, remote viewing, prophetic visions..etc Like the Evolution theory there is alot of conlusive evidence missing; Absent. But far too many people have experienced these phenomena to simply render them false/illusionairy.

Originally Posted by SKA

I certainly believe Abscence of evidence is NOT evidence of absence.

I'm sure that you don't.

I've already covered the general objection regarding objective (or absolute) certainty--scientific or otherwise--earlier in this thread in some detail, so rather than lapsing into repeating myself, I invite you to read those posts. I know it's a lot to read; I can be a bit long winded, but I invested quite a bit of time into addressing these exact points and I hardly think it's fair to me for you to simply skip over all that and demand that I explain myself over again from scratch. Let me know if you come back with specific rebuttals to what I've written, otherwise we're talking past each other.

I don't even know what you're trying to get across with your evolution example. As you say yourself, there is an absolute abundance of evidence in support of the theory in its broad form, so it's hardly a candidate for these "absence of evidence" type of arguments in the first place.

And please, don't make this about Big Bad Science. This thread has very little to do with science. It's about a neat little formula that tells us how to evaluate and revise our beliefs in a normative and personally consistent fashion. Nothing more, nothing less. An interesting and necessary consequence of this personal consistency happens to be that, if presence of evidence is going to be taken as evidence of presence, then absence of evidence must be taken as evidence of absence.

DuB is right -- this is not science, this is logic alone.

This stuff is really confusing. I think I get the gist of it, though I'll probably have a hard time explaining it to other people

It may help (both for understanding and for explanation purposes) if you simply skip over the probability terms I kept referring to and just think of the situation in terms of the cell phone example. I think it's a good example because it's one that is very familiar for most of us, and this familiarity nicely illustrates that, on some level, people really do intuitively understand that absence of evidence has to be evidence of absence. If they really didn't believe this was the case, why would they bother to call the cell phone at all? The fact that people do call their phones when in this situation reveals that they believe, implicitly or explicitly, that the presence or absence of the ring (the evidence) is going to be informative one way or another as to where the phone is (the hypothesis). You can think of it as "cell phone logic." All empirical knowledge is a matter of cell phone logic.

The purpose of referring to the probability terms and to Bayes' Rule at all is partly for the sake of brevity (it's a lot easier and, I think, less confusing to say "P(~E|H)" than it is to say "the probability of not observing the evidence, given that the hypothesis is true") and partly to demonstrate that there is a mathematical and axiomatically true basis to the argument. Even while referring to these terms, I still actually simplified quite a lot. I didn't plug in any actual values--indeed, I didn't even present the formula itself!--and I gave only superficial treatment to the issue of base rates (priors), which is a highly significant factor in this sort of analysis, but as I mentioned in the OP, not one that fundamentally alters the nature of the conclusions. Anyway, if you can wrap your head sufficiently around that part of the argument, more power to you, but as I noted above, the idea simply has intuitive appeal to most people. Even if they don't explicitly realize it or even acknowledge it, they do abide by it.

That does clear things up a bit DuB, thanks for taking the time to explain it to me again

I'll have to take another look at the explanation, that involves symbols sometime later.

DuB: good points as always, but I don't think your conjecture is true in all instances.

What about when you can't really assign a probability to something?

For instance,

P = universe has existed forever
¬P = universe has existed for infinite amount of time

Consider the situation before we had the equipment to check for evidence. You can't assign P a probability because you have no information or logical argument either way. Obviously though it would be wrong to say the absence of evidence for P is evidence that P is wrong. By symmetry you could say the same for ¬P, which is clearly absurd.

In mathematics, absence of proof can be proof of absence.

I don't think this applies to anything in mathematics actually. Proof of absence is proof of absence, and that alone, as it were.

The absence of solutions to Fermat's equation were not proven by the failure of anybody to find some, they were proven by Andrew Wiles via algebra.

Absolutely it's not true in every instance, which I made sure to mention in the OP (near the end: "This is true for any set of hypotheses where the correlation between E and H is different from 0, in other words, when evidence is actually evidence"). There are some situations for which there are no justifiable logical inferences to be made about the hypotheses, regardless of whether we view it from a Bayesian perspective or any other. This is another inadequacy of the "absence of evidence" line: it may be a clever turn of phrase, but it's terribly misleading because it conflates two very different ways in which evidence can be "absent."

On the one hand, we can have a reasonably clear idea about what the evidence might look like and which hypothesis would have given rise to the evidence in the most straightforward way, but when we go to look for this evidence, it isn't there. Then whichever hypothesis would have most straightforwardly caused the evidence to be absent becomes more probable at the expense of the other hypotheses. This is what the cell phone example illustrates, and this is almost always the type of situation to which people are referring when they invoke the "absence of evidence" line.

But on the other hand, evidence can be absent because (a) we don't have even an inkling as to what the evidence might look like, so we aren't sure what we should even look for, or (b) we do have an idea about what it may look like, but we haven't looked yet! Essentially, there isn't even any evidence to be absent. This is apparently the situation for the hypotheses concerning the existence of the universe. Before proper measurement was possible, evidence was "absent" in that there didn't seem to be any empirical observations that would support one hypothesis or the other, but this is clearly very different from the previous situations. Drawing the analogy back to the cell phone example, this would be like trying to draw an inference about the phone's location before I even tried calling the phone. Clearly absence of a ring is not informative in that case, because I haven't even looked for the ring yet!

The difference is that in this latter class of situations, the correlation between the "evidence" and the hypothesis is 0. Not hearing a ring coming from my bedroom is equally likely under both of the phone location hypotheses in this case because I haven't called the phone yet. Similarly, if we don't have a proper means of measurement that would allow us to infer things about how long the universe has existed, then not observing those measurements is equally likely regardless of how old the universe actually is; E and H are not correlated. The phrase about "absence of evidence" should not be applied to these cases in either its negative or its positive form, because absence of evidence implies that there is possible evidence which happens to be absent. However, if a certain piece of "evidence" is equally likely regardless of which hypothesis is true, it doesn't make any sense to call it "evidence" in the first place, and in fact I would say that it is by definition not evidence. It's simply irrelevant.

You are right that at least for me the terms evidence and proof are really interchangable. I see that in logic they arnt. In everyday speech it seems most people do see them as such. Im gonna seem like a grammer nazi if I start correcting people on that LOL