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"You have no evidence that there is no teapot on the moon, so there is one" - an argument from ignorance

Argument from ignorance, also known as argumentum ad ignorantiam or appeal to ignorance, is an informal logical fallacy. It asserts that a proposition is necessarily true because it has not been proven false (or vice versa). This represents a type of false dichotomy in that it excludes a third option: there is insufficient investigation and the proposition has not yet been proven to be either true or false.[1] In debates, appeals to ignorance are sometimes used to shift the burden of proof.

Carl Sagan famously criticized the practice by referring to it as "impatience with ambiguity", pointing out that "absence of evidence is not evidence of absence". This should not, however, be taken to mean that one can never possess evidence that something does not exist; an idea captured by philosopher Bertrand Russell's teapot.


Basic argument

Arguments that appeal to ignorance rely merely on the fact that the veracity of the proposition is not known, or is undetected, to arrive at a definite conclusion. These arguments fail to appreciate that the limits of one's understanding or certainty do not change what is true. This fallacy can be very convincing and is considered by some[2] to be a special case of a false dilemma or false dichotomy in that they both fail to consider perfectly valid alternatives. A false dilemma may take the form:

  • If a proposition has not been disproven, then it cannot be considered false and must therefore be considered true.
  • If a proposition has not been proven, then it cannot be considered true and must therefore be considered false.

Such arguments attempt to exploit the facts that (a) true things can never be disproven and (b) false things can never be proven. In other words, appeals to ignorance claim that the converse of these facts are also true (therein lies the fallacy).

To reiterate, these arguments ignore the fact, and difficulty, that some true things may never be proven, and some false things may never be disproved with absolute certainty. The phrase "absence of evidence is not evidence of absence" can be used as a shorthand rebuttal to the second form of the ignorance fallacy (i.e. P has never been absolutely proven and is therefore certainly false.). Most often it is directed at any conclusion derived from null results in an experiment or from the non-detection of something. In other words, where one researcher may say their experiment suggests evidence of absence, another researcher might argue that the experiment failed to detect a phenomenon for other reasons.

Matters of confusion

Much confusion about 'arguments from ignorance' can be caused when one side of a debate forgets that we often possess evidence of absence in practice.

The ignorance fallacy is sometimes confused (or combined) with logically valid contrapositive arguments. Contrapositive arguments rightly utilize the transposition rule of inference in classical logic to conclude something like: To the extent that C implies E then Not-E must also imply Not-C. In other words, if a cause always leads to an effect, then absence of the expected effect is evidence of absence of the cause. For example, if the causal proposition that If it's raining outside then the streets will be wet is assumed, then it can be assumed that if the streets are not wet then it is not raining outside. The inference that it cannot be raining outside because the streets are not getting wet is exactly as true, or perhaps exactly as untrue, as the original proposition. The statements are logically equivalent.

File:Carl Sagan Viking.JPG

Carl Sagan beside a Viking model

As Carl Sagan explains:

"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."-The Demon-Haunted World: (Chapter 12 - The Fine Art of Baloney Detection.)

For instance, absence of evidence that it rained (i.e. water is the evidence) may be considered as positive evidence that it did not rain. Again, in science, such inferences are always made to some limited (sometimes extremely high) degree of probability.

Arguments from ignorance can easily find their way into debates over the Existence of God. This, both from the theistic side (e.g. "You don't have evidence that my God doesn't exist, so regardless of my evidence - he exists!") and from the atheistic side (e.g. "You don't have evidence that your God exists, therefore he doesn't exist, regardless of whether I actually possess Evidence of absence"). Again, it is important to note that it is a fallacy to draw conclusions based precisely on ignorance, since this does not satisfactorily address issues of philosophic burden of proof. In other words, a complete lack of evidence either way results in agnosticism, thus each side must prove that they have satisfied their own burden for providing proof (evidence).

Related terms

Contraposition and Transposition

Contraposition is a logically valid rule of inference that allows the creation of a new proposition from the negation and reordering of an existing one. The method applies to any proposition of the type If A then B and says that negating all the variables and switching them back to front leads to a new proposition i.e. If Not-B then Not-A that is just as true as the original one and that the first implies the second and the second implies the first.

Transposition is exactly the same thing described in a different language.

Absence of evidence

Absence of evidence is the absence, or lack of, any kind of evidence that may show, indicate, suggest, or be used to infer or deduce a fact.

Evidence of absence

Evidence of absence is evidence of any kind that can be used to infer or deduce the non-existence or non-presence of something. For instance, if a doctor does not find any malignant cells in a patient this null result (finding nothing) is evidence of absence of cancer, even though the doctor has not actually detected anything per se. Such inductive reasoning is important to empiricism and science, but has well established limitations. The challenge thus becomes to try to identify when a researcher has received a null result (found nothing) because the thing does not exist (evidence of absence), and when one simply lacks proper means of detection (absence of evidence).

Negative evidence

Negative evidence is sometimes used as an alternative to absence of evidence and is often meant to be synonymous with it. On the other hand, the term may also refer to evidence with a negative value, or null result equivalent to evidence of absence. It may even refer to positive evidence about something of an unpleasant nature.

Null result

Null result is a term often used in the field of science to indicate evidence of absence. Keeping with the example above, a search for water on the ground may yield a null result (the ground is dry); therefore, it probably did not rain.

Related arguments

Argument from incredulity / Lack of imagination

Arguments from incredulity take the form:

  1. P is too incredible (or I cannot imagine how P could possibly be true); therefore P must be false.
  2. It is obvious that P (or I cannot imagine how P could possibly be false) therefore P must be true.

These arguments are similar to arguments from ignorance in that they too ignore and do not properly eliminate the possibility that something can be both incredible and still be true, or appear to be obvious and yet still be false.

Argument from self-knowing (auto-epistemic)

Arguments from self-knowing take the form:

  1. If P were true then I would know it; in fact I do not know it; therefore P cannot be true.
  2. If P were false then I would know it; in fact I do not know it; therefore P cannot be false.

In practice these arguments are often fallacious and rely on the veracity of the supporting premise. For example the argument that If I had just sat on a wild porcupine then I would know it; in fact I do not know it; therefore I did not just sit on a wild porcupine is probably not a fallacy and depends entirely on the veracity of the leading proposition that supports it. (See Contraposition and Transposition in the Related terms section in this article.)

Distinguishing absence of evidence from evidence of absence

Absence of Evidence is a condition in which no valid conclusion can be inferred from the mere absence of detection, normally due to doubt in the detection method. Evidence of absence is the successful variation: a conclusion that relies on specific knowledge in conjunction with negative detection to deduce the absence of something. An example of evidence of absence is checking your pockets for spare change and finding nothing but being confident that the search would have found it if it was there.

Formal argument

By determining that a given experiment or method of detection is sensitive and reliable enough to detect the presence of X (when X is present) one can confidently exclude the possibility that X may be both undetected and present. This allows one to deduce that X cannot be present if a null result is received.

Thus there are only two possibilities, given a null result:

  1. Nothing detected, and X is not present.
  2. Nothing detected, but X is present (Option eliminated by careful research design).

To the extent that option 2 can be eliminated, one can deduce that if X is not detected then X is not present and therefore the null result is evidence of absence.


Absence of evidence

(These examples contain or represent missing information.)

  • Statements that begin with "I can't prove it but…" are often referring to some kind absence of evidence.
  • "There is no evidence of foul play here" is a direct reference to the absence of evidence.

Negative results

  • When the doctor says that the test results were negative, it is usually good news.
  • Under "Termites" the inspector checked the box that read "no".
  • The results of Michelson–Morley's experiment reported no shift at all in the interference pattern.

Evidence of absence

(These examples contain definite evidence that can be used to show, indicate, suggest, infer or deduce the non-existence or non-presence of something.)

  • A biopsy shows the absence of malignant cells.
  • The null result found by Michelson–Morley's famous experiment represents "strong evidence" that the luminiferous aether was not present.
  • One very carefully inspects the back seat of one's car and finds no tigers.
  • The train schedule does not say that the train stops here at 3:00pm on a Sunday.

Arguments from ignorance

(Draws a conclusion based on lack of knowledge or evidence without accounting for all possibilities)

  • "I take the view that this lack (of enemy subversive activity in the west coast) is the most ominous sign in our whole situation. It convinces me more than perhaps any other factor that the sabotage we are to get, the Fifth Column activities are to get, are timed just like Pearl Harbor... I believe we are just being lulled into a false sense of security." - Then California's Attorney General Earl Warren (before a congressional hearing in San Francisco on 21 February 1942)

In the field of science

  • One look in the back seat of one's car and finds no adult-sized kangaroos and then uses this negative/null adult-sized kangaroo detection results in conjunction with the previously determined fact (or just plain old proposition) that adult-sized kangaroos, if present, cannot evade such detection, to deduce a new fact that there are indeed no adult-sized kangaroos present in the back seat of said car.

Principles in law

  • The presumption of innocence, if present, effectively removes the possibility that the accused may be both guilty and unproven, from consideration in judgment, and as such the accused is considered as innocent unless proven guilty. (See decision table below)
    1. Innocent and unproven. Judged as innocent.
    2. Innocent and proven. Judged as innocent.
    3. Guilty and unproven. Judged as innocent. (Presumption of innocence)
    4. Guilty and proven. Judged as guilty. (Innocent unless/until proven guilty is a summary of this and easier to remember.)

Origin of the term

From "Fallacies: classical and contemporary readings By Hans V. Hansen, Robert C. Pinto"

"It is generally accepted that the philosopher John Locke introduced the term in his Essay Concerning Human Understanding:"
"Another way that Men ordinarily use to drive others, and force them to submit their Judgments. And receive the Opinion in debate, is to require the Adversary to admit what they alledge [sic] as a Proof, or assign a better. And this I call Argumentum ad Ignorantum" - John Locke


  • Fallacies: classical and contemporary readings By Hans V. Hansen, Robert C. Pinto
  • Introduction to Logic by Irving Marmer Copi.
  • Essay Concerning Human Understanding Book IV - John Locke

See also


External links

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