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 data 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.

General forms of the argument:

1. P has never been disproven therefore P is/(must be) true.
2. P has never been proven therefore P is/(must be) false.

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 (one can possess such evidence). Instead, Sagan's famous quote is a reminder that inferences must be made carefully, and that science makes no claims to absolute certainty, only high probability.

Overview

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 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 can't be considered false, therefore it must be considered true.
• If a proposition has not been proven then it can't be considered true, therefore it must be considered false.

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.

This fallacy is sometimes confused , and or combined, with logically valid contrapositive arguments. Contrapositives rightly utilize the transposition rule of inference in classical logic to conclude that: 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 we assume the causal proposition that If it's raining outside then the streets will be wet, then we can reason 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.

Carl Sagan's Contribution

The phrase "absence of evidence is not evidence of absence" can be used as a short hand 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.

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

"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."

In this regard Irving Marmer Copi writes:

"In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence as positive proof of its non-occurrence." (Introduction to Logic, Copi, 1953, Page 95)

Therefore, 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.

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 if you negate all the variables and switch them back to front you will get a new proposition i.e. If Not-B then Not-A that is just as true as the one you started out with and that the 1st implies the 2nd and the 2nd implies the 1st.

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 shows, indicates, suggests, or can be used to infer or deduce the non-existence or non-presence of something. In some sense "absence of evidence is sometimes evidence of absence". For instance, absence of evidence that there are malignant cells is evidence of absence that there is cancer.

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 can't imagine how P could possibly be true) therefore P must be false.
2. It's obvious that P (or I can't 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 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 where 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 un-detected and present. This allows us to deduce that X cannot be present if we receive a null result.

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.

Examples

Absence of evidence

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

• A biopsy showing 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.
• You very carefully inspect the back seat of your car and lo… 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

• You look in the back seat of your car and lo... no adult sized kangaroos and then use 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. (It's a lot of trouble being rigorous.)

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 accept 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

Sources

• 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

• Negation as failure
• Argument from silence
• Indeterminacy problem
• False dilemma

References

External links

• LingLogic" Appeal to ignorance
• Fallacy Files article on Appeal to Ignorance