Dhorpatan's logic of absence IS absence of logic.

The absence of evidence can be used to support an argument, but only in the right circumstances. I explore those possibilities in this video response to the inane rhetoric of Dhorpatan.

"You cannot cite absence of evidence in the rejection of a claim, without first establishing that the truth of the claim would necessitate our knowledge of the evidence that we, indeed, lack."

http://www.youtube.com/watch?v=XNRftzOWtxg

http://en.wikipedia.org/wiki/Modus_tollens

Category:

Education

Tags:

• logic
• modus
• tollens
• absence
• of
• evidence
• dhorpatan

[iamgoddard]

A fast logical analysis concurs with your conclusion:

If absence of evidence is evidence of absence,

then: no evidence = evidence.

But that's a contradiction of identity, -A =/= A.

Therefore, absence of evidence is NOT evidence of absence (or of anything).

[IAmTheScum13]

@iamgoddard That's not quite right, because in this case, "evidence" is referencing two different things.

For example: If my right hand is not dominant, then my left hand is dominant. This isn't actually true, but let's roll with it. "Dominance" references two different hands, as "evidence" before is referencing two different propositions.

[iamgoddard]

Agree with your conclusion, but differ on this:

That "there is no evidence for the dollar" in your hand (as you say) does not adequately model the given state of affairs. A better modeling is: There IS visual evidence for the nonexistence of a dollar in your hand.

So the example is not an example of 'absence of evidence', and hence Sagan's axiom, "An absence of evidence is NOT evidence of absence," is not shown to be false sometimes (and you seem to say it is false sometimes).

[IAmTheScum13]

@iamgoddard But then I would ask, what is the evidence for the nonexistence of the dollar, and you would cite your lack of visual perception of the dollar. Visual perception of the dollar would count as evidence for the existence of the dollar, therefore you lack evidence.

[Dhorpatan]

Here's how the Absence of evidence works. if you make a CLAIM that there is a dollar bill in your desk drawer; but you never present any valid objective evidence to back up that claim, that absence of evidence for your claim, is evidence that your claim is absent of truth. And this is reasonable until otherwise refuted. Its evidence your claim is false, IT DOES NOT PROVE IT.

[IAmTheScum13]

@Dhorpatan And that leaves us with an interesting dichotomy. We have two claims.

There is a dollar bill in my in my desk drawer.

There is no dollar bill in my desk drawer.

Both lack evidence. So by your standard, it's reasonable to believe their negations (which result in the same two initial claims) simultaneously. You are falling into the absurd reasoning I highlighted in the second half of the vid.

[BrooklynRagtag]

Other than an absence of confirming evidence, what evidence could you possibly cite in favor of the non-existence of a thing? How would you demonstrate it?

[jonThedrummerify]

Interesting video. However, I think the dollar bill in the drawer argument is not analogous to the argument that an immaterial/eternal/omnipotent/­omnipresent being or deity does or doesn't exist, because it is not against the laws of space, time, and matter for a dollar bill to be in that drawer. An eternal, all knowing, and immaterial being is, however, in direct violation of the laws of our universe. That is a very important thing to take into account.

[adavidpirtle]

There is evidence that the dollar bill is not in your hand.

[Birdieupon]

@Pelonetillo

"the likelihood in question is that of the claim being true; but youre focused on the likelihood of the coin landing a certain way"

What the hell are you talking about? According to Dhorpatan, AoE for a claim makes its negation MORE LIKELY to be true. This would mean if I said "it's heads" then his faulty logic would imply I should be more inclined to believe that the coin landed tails (i.e. that "heads" is not true). It's not even clear what your objection is? Maybe its just BS.

[Pelonetillo]

you didnt get it.

the likelihood in question is that of the claim being true; but youre focused on the likelihood of the coin landing a certain way--wrong focus.

[Birdieupon]

@Pelonetillo

""more likely" relative to what?"

I'm merely quoting the words of Dhorpatan himself, who said absence of evidence for a claim makes its negation "more likely" and pointing out this is fallacious.

[Pelonetillo]

"more likely" relative to what? given the context youve provided in your comment, there is more than one answer to that question. youre focused on one; thats why youre confused.

[Birdieupon]

@Dhorpatan

"Its evidence your claim is false, IT DOES NOT PROVE IT."

Using caps won't make your point any more valid. What would you say in response to the claim "the coin I tossed landed heads" but you couldn't see the coin? Would you conclude that the AoE made it more likely that it was tails? It's clearly absurd.

You're right: it doesn't prove it, but neither does it tell us *anything* about the likelihood of its truth value. You'd need a modus tollens argument when dealing with absence.

[Birdieupon]

@Dhorpatan

"if no evidence for that claim is given(when evidence should be expected or is asked for), then that is evidence that your claim has an absence of truth."

Dude, what if the claim was "I do NOT have a dollar bill in my drawer"? By your logic, the AoE would make it more likely that he doesn't have it either! You talk about evidence being expected, but that's the point: you're asking for evidence to find the truth, not relying upon the absence of it. You've given no valid refutation.