Is Absence of Evidence ever Evidence of Absence?

[Consul]

+++This is an off-topic split-off from the thread Can consciousness exist without any brain at all?!+++

As I already explained in previous posts, if there should be evidence for p if p is true, then absence of evidence amounts to evidence of absence.—…then absence of evidence for p is evidence for ~p.

An absence of evidence for p is an absence of evidence. How does zero evidence for p become positive evidence for ~p?

What does 'should' mean in "there should be evidence of p"?

"used in auxiliary function to express what is probable or expected" (Merriam-Webster)

In this sense, for example, if there is milk in your fridge, there should be perceptual evidence for it that you can obtain by opening its door and looking inside, since milk is easily visible stuff. And if you do so (thoroughly) without finding any (positive) perceptual evidence for the presence of milk, then this is (negative) perceptual evidence for the absence of milk, and you are thereby justified in believing that there is no milk in your fridge.

If, for a particular p, "p is true" implies "there is evidence for p", then zero evidence for p implies p is not true.

Does this logical conclusion based on zero evidence and the precondition qualify as positive evidence for ~p? Accepting that p is true could be based on evidence or based purely on logic without any evidence. Does logic qualify as evidence or is logic beyond evidence?

What is needed for "p is true" to imply "there is evidence for p"?

Perhaps this depends on what qualifies as evidence?

The central form of evidence in empirical science is perceptual evidence in the form of perceptual experience; and there's a distinction between things or facts which are perceptually accessible (in practice or in principle at least) and ones which are not. Of course, there cannot be any perceptual evidence for perceptually inaccessible, imperceptible things or facts; but imperceptibility and a corresponding lack of perceptual evidence don't entail nonexistence, since—pace George Berkeley—it is not the case that being is being perceived.

So the absence of evidence for a thing or fact is evidence for its absence if and only if it is perceptually accessible (directly or indirectly at least), and no perceptual evidence for it has been found—provided the search for evidence was performed non-superficially, painstakingly, i.e. with great care and attention.

For example, if there is milk in your fridge, but it is "magic milk" that becomes invisible as soon as you open its door, then the absence of visual evidence for it is not evidence for its absence if you know that it becomes invisible every time the door is opened. For then you cannot expect there to be visual evidence for it.

If p is a statement regarding the physical existence of X, then if X exists it may be assumed there will be some kind of indication in the universe that X exists which could qualify as evidence. Does this "evidence" need to be discovered and/or possessed before it qualifies as evidence?

There is no perceptual evidence for something before somebody had some perceptual experience of it. For instance, I have no visual or auditory evidence for there being a fly in my living room unless I actually see or hear a fly in my living room. My seeing or hearing (or my seeming to see or hear) a fly is my evidence for there being a fly.

But the term "evidence" isn't always used to refer to perceptual/experiential evidence (in the form of perceptual experience). Especially in the legal context, physical items or signs are called evidence, and such physical evidence exists before it is perceived by somebody (as opposed to mental evidence or "phenomenal evidence" in the form of perceptual experience):

"When one compares philosophical accounts of evidence with the way the concept is often employed in non-philosophical contexts, however, a tension soon emerges. Consider first the kinds of things which non-philosophers are apt to count as evidence. For the forensics expert, evidence might consist of fingerprints on a gun, a bloodied knife, or a semen-stained dress: evidence is, paradigmatically, the kind of thing which one might place in a plastic bag and label ‘Exhibit A’. Thus, a criminal defense attorney might float the hypothesis that the evidence which seems to incriminate his client was planted by a corrupt law enforcement official or hope for it to be misplaced by a careless clerk. For an archaeologist, evidence is the sort of thing which one might dig up from the ground and carefully send back to one's laboratory for further analysis. Similarly, for the historian, evidence might consist of hitherto overlooked documents recently discovered in an archive or in an individual's personal library. Reflection on examples such as these naturally suggests that evidence consists paradigmatically of physical objects, or perhaps, physical objects arranged in certain ways. For presumably, physical objects are the sort of thing which one might place in a plastic bag, dig up from the ground, send to a laboratory, or discover among the belongings of an individual of historical interest."

Source: https://plato.stanford.edu/entries/evidence/

If Mary places 51 playing cards face down on a table and says to John, "p is: one of these cards is the Queen of Hearts", it could be argued: if p is true then there is evidence of p on the table; John is just unable to view this evidence.

Initially John only sees 51 cards face down on the table. He has no evidence for p and no evidence for ~p. Relative to his lack of evidence, p and ~p are both possible. What can he conclude?

If John turns a card over, this reveals something. This provides him with some evidence. If this card is the Queen of Hearts, this is conclusive evidence for p. If this is another card then this may provide a little bit of evidence for ~p.

If he turns 50 cards over and none of these are the Queen of Hearts, this may provide a lot of evidence for ~p. If p is true, the chances are he would have uncovered conclusive evidence for this by now. But one card remains unturned. This may or may not be the Queen of Hearts. How can he usefully calculate the probability that p is true (or not true) at this stage?

If he turns over the 51st card and this is also not the Queen of Hearts, he may finally have conclusive evidence for ~p (assuming no card trickery).

In order have conclusive evidence for absence of X in Y, Y (eg, 51 cards on the table) needs to be finite, John's examination of Y needs to be exhaustive (he needs to have full access to Y and not leave any card unturned) and he needs to be able to tell the difference between X (Queen of Hearts) and ~X.[/quote]

If John knows Mary didn't lie, he knows (after having turned over 50 cards) that the 51st card is the Queen of Hearts before turning it over, since there is no other possibility. He doesn't know this in advance if he doesn't know whether Mary lied or not.

As I already remarked above, the search for evidence must be performed painstakingly so as not to make the mistake of failing to notice evidence that is there.

If Y is the universe (or beyond the universe) then obtaining conclusive evidence for absence of X may be very challenging.

Of course, if the search area is the whole universe, then we'll never be justified in claiming e.g. that there is nowhere evidence for extraterrestrial life; and then we'll never be justified in claiming that there is no extraterrestrial life anywhere in the universe.

If John is not able to turn over any cards, he may not be able to obtain any evidence of p or ~p. However, if Mary says, "I know what the cards are; all the evidence you need is in my mind if you can access this", how can John access this evidence? If Mary is a robot, he may be able to access her internal data. If Mary is human he may able to intercept and decode her brain waves.
If Mary says, "p is: I, Mary, have phenomenal consciousness", how can John begin to obtain evidence for p, or evidence for ~p?

That's the good old epistemic question of other minds/consciousnesses: I know I am conscious, but how do I know whether you are too?

Other Minds: https://plato.stanford.edu/entries/other-minds/

[Sculptor1]

+++This is an off-topic split-off from the thread Can consciousness exist without any brain at all?!+++

As I already explained in previous posts, if there should be evidence for p if p is true, then absence of evidence amounts to evidence of absence.—…then absence of evidence for p is evidence for ~p.

An absence of evidence for p is an absence of evidence. How does zero evidence for p become positive evidence for ~p?

What does 'should' mean in "there should be evidence of p"?

"used in auxiliary function to express what is probable or expected" (Merriam-Webster)

In this sense, for example, if there is milk in your fridge, there should be perceptual evidence for it that you can obtain by opening its door and looking inside, since milk is easily visible stuff. And if you do so (thoroughly) without finding any (positive) perceptual evidence for the presence of milk, then this is (negative) perceptual evidence for the absence of milk, and you are thereby justified in believing that there is no milk in your fridge.

A fridge is a closed system. A good investigation produces evidence of absense. There is no need to posit absence of evidence. This example is not valid. If the fridge door remains closed then we have absence of evidence. Only when we open it can we know if there is dead cat in there, or a cat that has drunk the milk.

If, for a particular p, "p is true" implies "there is evidence for p", then zero evidence for p implies p is not true.

Does this logical conclusion based on zero evidence and the precondition qualify as positive evidence for ~p? Accepting that p is true could be based on evidence or based purely on logic without any evidence. Does logic qualify as evidence or is logic beyond evidence?

What is needed for "p is true" to imply "there is evidence for p"?

Perhaps this depends on what qualifies as evidence?

The central form of evidence in empirical science is perceptual evidence in the form of perceptual experience; and there's a distinction between things or facts which are perceptually accessible (in practice or in principle at least) and ones which are not. Of course, there cannot be any perceptual evidence for perceptually inaccessible, imperceptible things or facts; but imperceptibility and a corresponding lack of perceptual evidence don't entail nonexistence, since—pace George Berkeley—it is not the case that being is being perceived.

So the absence of evidence for a thing or fact is evidence for its absence if and only if it is perceptually accessible (directly or indirectly at least), and no perceptual evidence for it has been found—provided the search for evidence was performed non-superficially, painstakingly, i.e. with great care and attention.

Yes but all you are saying here is that absence of evidence has no empirical content. Evidence of absence does not even mean that there is certainty, unless you have a closed system and define those parameters. Inductively we can say I can't find it, I can't find it. But only when every stone is turned we can never say it does not exist. There can always be a black swan somewhere in the universe. It turns out they were in Australia all the time.

For example, if there is milk in your fridge, but it is "magic milk" that becomes invisible as soon as you open its door, then the absence of visual evidence for it is not evidence for its absence if you know that it becomes invisible every time the door is opened. For then you cannot expect there to be visual evidence for it.

But milk is defined as non-magical.

[Pattern-chaser]

In this sense, for example, if there is milk in your fridge, there should be perceptual evidence for it that you can obtain by opening its door and looking inside, since milk is easily visible stuff. And if you do so (thoroughly) without finding any (positive) perceptual evidence for the presence of milk, then this is (negative) perceptual evidence for the absence of milk, and you are thereby justified in believing that there is no milk in your fridge.

This is a good example. It is very highly constrained, and constructed to best illustrate your point. But you forgot to specify that the milk is not "magic milk", as you described later, and that the fridge is not sufficiently packed that its other contents could hide the milk from view. Such details go on and on, but they are all necessary to maintain the conclusion that absence of evidence is evidence of absence. What if the fridge light is off, and you're visiting the fridge in the dark of night? These ridiculous details go on and on, seemingly without end, and any one of them, if present, can refute your thesis. The constraints continue to the point where, as I have observed before, the example becomes a parody of itself, a contrived pantomime that is superficial and trivial.

[Consul]

This is a good example. It is very highly constrained, and constructed to best illustrate your point. But you forgot to specify that the milk is not "magic milk", as you described later, and that the fridge is not sufficiently packed that its other contents could hide the milk from view. Such details go on and on, but they are all necessary to maintain the conclusion that absence of evidence is evidence of absence. What if the fridge light is off, and you're visiting the fridge in the dark of night? These ridiculous details go on and on, seemingly without end, and any one of them, if present, can refute your thesis. The constraints continue to the point where, as I have observed before, the example becomes a parody of itself, a contrived pantomime that is superficial and trivial.

No, your description is a caricature, because there is no endless number of conditions that must be met before absence of evidence becomes evidence of absence.

QUOTE>
"If we take arguments for the existence of something to include all the evidence which supports the existence claim to any significant degree, i.e. makes it at all probable, then the absence of such evidence means there is no likelihood of the existence of the entity. And this, of course, is a complete justification for the claim that the entity does not exist, provided that the entity is not one which might leave no traces (a God who is impotent or who does not care for us) and provided we have comprehensively examined the area where evidence would appear if there were any."

(Scriven, Michael. Primary Philosophy. New York: McGraw-Hill, 1966. p. 102)

"A person is justified in believing that X does not exist if (1) all the available evidence used to support the view that X exists is shown to be inadequate; and (2) X is the sort of entity that, if X existed, there would be available evidence that would be adequate to support the view that X exists; and (3) the area where evidence would appear, if there were any, has been comprehensively examined."

(Martin, Martin. Atheism: A Philosophical Justification. Philadelphia: Temple University Press, 1990. p. 282)
<QUOTE

[3017Metaphysician]

This is a good example. It is very highly constrained, and constructed to best illustrate your point. But you forgot to specify that the milk is not "magic milk", as you described later, and that the fridge is not sufficiently packed that its other contents could hide the milk from view. Such details go on and on, but they are all necessary to maintain the conclusion that absence of evidence is evidence of absence. What if the fridge light is off, and you're visiting the fridge in the dark of night? These ridiculous details go on and on, seemingly without end, and any one of them, if present, can refute your thesis. The constraints continue to the point where, as I have observed before, the example becomes a parody of itself, a contrived pantomime that is superficial and trivial.

No, your description is a caricature, because there is no endless number of conditions that must be met before absence of evidence becomes evidence of absence.

QUOTE>
"If we take arguments for the existence of something to include all the evidence which supports the existence claim to any significant degree, i.e. makes it at all probable, then the absence of such evidence means there is no likelihood of the existence of the entity. And this, of course, is a complete justification for the claim that the entity does not exist, provided that the entity is not one which might leave no traces (a God who is impotent or who does not care for us) and provided we have comprehensively examined the area where evidence would appear if there were any."

(Scriven, Michael. Primary Philosophy. New York: McGraw-Hill, 1966. p. 102)

"A person is justified in believing that X does not exist if (1) all the available evidence used to support the view that X exists is shown to be inadequate; and (2) X is the sort of entity that, if X existed, there would be available evidence that would be adequate to support the view that X exists; and (3) the area where evidence would appear, if there were any, has been comprehensively examined."

(Martin, Martin. Atheism: A Philosophical Justification. Philadelphia: Temple University Press, 1990. p. 282)
<QUOTE

Hi Consul!

Nice OP. I had two quick thoughts.

1. Part of the narrative reminds me of the Unity of Opposites philosophy in that one cannot exist without the other. Much like hot/cold, dry/wet, happy/sad. Those concepts, in themselves, convey the notion of dependence on 'opposites' for them to make sense to us.

2. I noticed the quotes from 'A-theism' or perhaps better said, an atheistic type of belief system. To that end, I'm thinking that this aforementioned concept of Unity rears its head again here. Consider the concept of theism/a-theism. The A-theist, must base their belief on that 'evidence' and/or lack thereof. Of course, that in itself is problematic (relative to logic, belief, etc.) but don't want to necessarily derail the thread (more appropriate for Religious discussion I suppose).

[Pattern-chaser]

This is a good example. It is very highly constrained, and constructed to best illustrate your point. But you forgot to specify that the milk is not "magic milk", as you described later, and that the fridge is not sufficiently packed that its other contents could hide the milk from view. Such details go on and on, but they are all necessary to maintain the conclusion that absence of evidence is evidence of absence. What if the fridge light is off, and you're visiting the fridge in the dark of night? These ridiculous details go on and on, seemingly without end, and any one of them, if present, can refute your thesis. The constraints continue to the point where, as I have observed before, the example becomes a parody of itself, a contrived pantomime that is superficial and trivial.

No, your description is a caricature, because there is no endless number of conditions that must be met before absence of evidence becomes evidence of absence.

Hmm. And yet, in your example, every one of those detailed constraints must be specified and met before it demonstrates that absence of evidence is evidence of absence. Every one. If even one of them is omitted, your example falls down (from your POV).

In what way is my description a caricature? Is there even one constraint that I mentioned that makes it a caricature, and if so, which one?

[RJG]

Is Absence of Evidence ever Evidence of Absence?

NO. There can NEVER be a scenario where ~X=X. (Not-Evidence is Evidence).

In this sense, for example, if there is milk in your fridge, there should be perceptual evidence for it that you can obtain by opening its door and looking inside, since milk is easily visible stuff.

In this case, this is Evidence-of-"milk in the fridge".

And if you do so (thoroughly) without finding any (positive) perceptual evidence for the presence of milk, then this is (negative) perceptual evidence for the absence of milk, and you are thereby justified in believing that there is no milk in your fridge.

And in this case, this is Evidence-of-"no milk in the fridge". Note: the "absence-of-milk" is not absence-of-evidence, it is evidence-of-absence (i.e. the evidence of the absence of milk).

In neither case is there "Absence-of-Evidence". We have "evidence" in both cases; we have evidence of milk in the fridge in the first case, and we have evidence of no milk in the fridge in the second case.

...conditions that must be met before absence of evidence becomes evidence of absence.

There are absolutely NO conditions where absence-of-evidence could become evidence-of-absence.

[chewybrian]

There are absolutely NO conditions where absence-of-evidence could become evidence-of-absence.

So, no rational person could be an atheist, then?

[RJG]

There are absolutely NO conditions where absence-of-evidence could become evidence-of-absence.

So, no rational person could be an atheist, then?

If the atheist's reason (for the non-existence of God) was based on "lack of evidence", then the atheist would be committing an "appeal-to-ignorance" logical fallacy. This is non-rational.

[chewybrian]

There are absolutely NO conditions where absence-of-evidence could become evidence-of-absence.

So, no rational person could be an atheist, then?

If the atheist's reason (for the non-existence of God) was based on "lack of evidence", then the atheist would be committing an "appeal-to-ignorance" logical fallacy. This is non-rational.

My own ignorance might be showing. I agree with you about that particular reason for denying God, though it begs the question of what a rational argument would be. It seems rational to be agnostic, and even 100 to 1 against if that is the way you lean. We can neither prove nor disprove the existence of God, so there you go. But, if you say there is 0% chance that there is a God, how does that argument go? In other words, do you think a person can disprove God, or is it necessary to do so to rationally deny God (not just doubt, but be certain God does not exist)?

[RJG]

I agree with you about that particular reason for denying God, though it begs the question of what a rational argument would be.

There is no rational (logical) argument denying God's existence.

It seems rational to be agnostic…

Agreed.

We can neither prove nor disprove the existence of God, so there you go.

Bingo! ...the belief or disbelief in God takes an act of (blind) faith, either way.

[Steve3007]

Is Absence of Evidence ever Evidence of Absence?

If we're talking about the reasons why do or don't believe various things then yes, I propose that absence of evidence can constitute evidence of absence for the purpose of deciding what to believe or not to believe. But a common mistake, it seems to me, is to associate belief with, on the one hand, certainty and, on the other hand, blind faith. It is neither. Obviously absence of evidence for the existence of a particular thing (X) does not constitute proof that X doesn't exist anywhere. But it does provide justification for believing that it doesn't exist. Obviously beliefs can change if/when evidence does come to light.

So, taking the atheism example that has been mentioned: An atheist is a person who believes that God, as they understand the term, doesn't exist. Their justification for holding that belief can quite reasonably be that they haven't yet seen what they would regard as evidence for the existence of God. If we didn't hold beliefs like that then we would find ourselves believing in the existence of an infinite number of things just because we haven't seen evidence for their absence but have only failed to see evidence for their presence. Clearly we don't do that. For good reason.

[Steve3007]

In this sense, for example, if there is milk in your fridge, there should be perceptual evidence for it that you can obtain by opening its door and looking inside, since milk is easily visible stuff. And if you do so (thoroughly) without finding any (positive) perceptual evidence for the presence of milk, then this is (negative) perceptual evidence for the absence of milk, and you are thereby justified in believing that there is no milk in your fridge.

This a particular example of a search space (a fridge) which can be exhausted in a very short period of time and an object (milk) that can be unambiguously identified in a very short period of time. Both periods of time are near instantaneous. But not all situations are like that. It doesn't necessarily follow that we suspend belief in those situations. For example, take the proposition that there is an extant "missing link" species between two other species somewhere on the planet. We can (in my view) justifiably believe that the species in question does not currently exist because it has not been found. Obviously if it's found we'll change that belief. Beliefs can and do change in the light of new evidence. But that doesn't stop the belief from justifiably existing now.

[chewybrian]

I agree with you about that particular reason for denying God, though it begs the question of what a rational argument would be.

There is no rational (logical) argument denying God's existence.

It seems rational to be agnostic…

Agreed.

We can neither prove nor disprove the existence of God, so there you go.

Bingo! ...the belief or disbelief in God takes an act of (blind) faith, either way.

OK, we are on the same page. I know you are the king of logic, and I wondered if you might have an interesting take on that.

[3017Metaphysician]

I agree with you about that particular reason for denying God, though it begs the question of what a rational argument would be.

There is no rational (logical) argument denying God's existence.

It seems rational to be agnostic…

Agreed.

We can neither prove nor disprove the existence of God, so there you go.

Bingo! ...the belief or disbelief in God takes an act of (blind) faith, either way.

OK, we are on the same page. I know you are the king of logic, and I wondered if you might have an interesting take on that.

Chewy!

Just a footnote if I may. The irony is that the human mind itself (consciousness) can neither be explained rationally, or exist rationally. For instance, the conscious and subconscious mind working together violates the rules of non-contradiction/bivalence in a priori logic; it (the mind) would be considered logically impossible. Metaphorically and Existentially, this is also why human Beings who live life, at some point, usually find life as not just an a or b proposition. It's both. Engineering, on the other hand, is primarily ' a or b ' (apply the wrong formula to the structural beam and it fails-the design is either right or wrong). And in a funny way, this is why some people argue that it is harder for many engineers to be sensitive managers or people-persons. It's not really required for the job.


So, hate to take the wind out of one's sails, but living life isn't all that logical. Unless you believe in a Platonic existence