Absence of evidence fallacy:

Evidential Fallacy
Definition Example
When it is argued that finding no evidence for something is no evidence for the absence of that thing. The fact that you did not see me at your birthday party does not mean I was not there!
  Notes
 

The absence of evidence is not evidence of absence.

This argument, attributed to Carl Sagan, is often invoked when claims of a god comes under scrutiny. While the absence of evidence is not proof of absence, it is, to varying degrees, evidence of absence. The degree of evidence an absence of evidence is for the absence of anything will depend on the context. If there is no evidence of thing X in a location Y, the degree that will constitute evidence for the absence of that thing depends upon how we substantiate the variables. Is X a molecule, mouse or monster? Is Y a cup, couch or continent? The expected discoverability of X within its relevant logical or spacial space Y must be considered.


“If someone were to assert that there is an elephant on the quad, then the failure to observe an elephant there would be good reason to think that there is no elephant there. But if someone were to assert that there is a flea on the quad, then one’s failure to observe it there would not constitute good evidence that there is no flea on the quad. The salient difference between these two cases is that in the one, but not the other, we should expect to see some evidence of the entity if in fact it existed. Moreover, the justification conferred in such cases will be proportional to the ratio between the amount of evidence that we do have and the amount that we should expect to have if the entity existed. If the ratio is small, then little justification is conferred on the belief that the entity does not exist. [For example] in the absence of evidence rendering the existence of some entity probable, we are justified in believing that it does not exist, provided that (1) it is not something that might leave no traces and (2) we have comprehensively surveyed the area where the evidence would be found if the entity existed…”

—J.P. Moreland and W.L. Craig, Philosophical Foundations for a Christian Worldview


Case Study One

After a Harvard Medical School study on the effects of prayer on heart surgery patients showed no statistical advantage of prayer, it was wrongly argued that “the absence of evidence is not evidence of absence”.


Case Study Two

Those who claim they have the biblical Holy Spirit helping them to make predictions or avoid logical fallacies in discussion, when confronted with evidence that they are, at best, only on par with those who do not claim to have a Holy Spirit, they often invoke this absence of evidence fallacy. They then often claim the burden of proof falls on those denying the existence of their Holy Spirit.


Case Study Three

Donald Rumsfeld invoked the “absence of evidence” fallacy in reference to possible weapons of mass destruction in Iraq. Having not found WMDs where they were expected was, in fact, evidence of their absence.


Keep in mind that a fallacious argument does not entail an erroneous position.


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6 Responses to Absence of evidence fallacy:

  1. James says:

    For more info on this, google for proof that absence of evidence is evidence of absence using conditional probability.

    To summarize what it says, absence of evidence IS evidence of absence, but it is not PROOF of absence or even necessarily STRONG evidence.

    Usually, absence of evidence is only WEAK evidence of absence.

    Most confusion on this comes from people mixing up the meanings of proof and evidence, as well as how strong the evidence is. So for clarification, here’s the meanings of the terms used in the statement.

    Proof:
    Offers complete certainty. (think ala math or deductive logic proof) .

    Evidence:
    Increases certainty, but does not render it complete (think as in probability or inductive arguments).

    • Todd says:

      The above is not sound and actually commits the fallacy. You cannot use an absence of something as evidence in your favor.

      • Todd, here are a few additional examples in which the absence of evidence is indeed evidence of absence.

        1. The man who has been living next door for 10 years claims to have a wife living with him. To the degree (length of time living there and amount of observation) that you have not seen a woman next door, to that degree you are warranted in doubting his claim.

        2. A female friend claims to be 8 months pregnant. To the degree she does not exhibit the round belly nomally associated with pregnant women, to this degree you have evidence to doubt the claim.

        3. A man claiming to have clairvoyance submits to a test of his claim, but fails to identify the objects printed on the hidden side of cards above what would be expected from chance. The absence of success above chance constitutes evidence that he lacks clairvoyance.

        To the degree that there is an absence of effects normally associated with the presence of a given entity or state of existence, to that degree we can consider that lack of confirming evidence disconfirming evidence.

    • Brian P. Rabbit says:

      The question of “Does AOE = EOA?” depends upon the reproducibility of AOE. If 1 Person does not see Me at a party, it does not mean Other did not and, yes, the argument would then be “weak”. However, if Nobody at that party saw Me, the likelihood I was not there increases. Applying the “AOE = EOA” argument to the existence of any particularly alleged Deity is quite possibly fallacious in itself, however, because alleged Deities to which the “AOE = EOA” is applied are, in My experience, Those which allegedly have the same degree of choice which Any Other Person has and the tests for which “AOE = EOA” is applied are, in My experience, often those which would require the alleged Deity to actively participate. Since Everyone is often free to choose whether or not to actively participate in tests, testing an alleged Deity which has that same freedom to choose whether or not to actively participate is about as logically sound as saying, “Because Thomas Shmalahazen didn’t show up to participate in this test, Thomas must not exist.” However, if I understand correctly, such a fallacy is a different one than the one listed on this page.

      • Any volitional and omnipotent god can choose to hide, but the definition of that god must then include the notion that this god chooses to hide. Only then can the apparent absence of that god be consistent with its existence.

        But even when a child is playing hide-and-seek with a “friend” whom their parents has not previously seen, there comes a point in their search around the “house” at which your parents are justified in concluding that your “friend” most likely never existed.

  2. Pingback: Feminist theologian: Jesus may have been a hermaphrodite | Fellowship of the Minds

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