Argument from Ignorance

An argument from ignorance, also known as ‘argumentum ad ignorantiam’ or ‘appeal to ignorance’ (where ‘ignorance’ stands for: ‘lack of evidence to the contrary’), is an inference that a proposition is false based on the absence of evidence.

For example, a man sitting in a warehouse with a tin roof can assume that it is not raining if he doesn’t hear rain drops without looking outside for any evidence of rain. Here ignorance about a particular form of evidence for rain (the noise) is used to assume a lack of rain; but the conclusion may fail if it is raining so softly that no noise is heard by the man, or if his hearing is impaired, etc.

The statement ‘absence of evidence is not evidence of absence’ has been used in fields such as epistemology, medical research, archaeology and criminology, where it is taken as an axiom that the lack of evidence that a person was present at a scene does not imply that they were not there.

Philosopher John Locke introduced the term ‘argumentum ad ignorantiam’ in 1689 in ‘An Essay Concerning Human Understanding,’ and stated: ‘Secondly, Another way that men ordinarily use to drive others and force them to submit to their judgments, and receive their opinion in debate, is to require the adversary to admit what they allege as a proof, or to assign a better. And this I call ‘argumentum ad ignorantiam.” Locke presented this type of argument as a form of eristic (pointless and hostile) debate in which participants wrangle over the issues without substance in evidence, but to ‘drive each other’ to accept a point.

The statement ‘absence of evidence is not evidence of absence’ was coined by astronomer Martin Rees, and made popular by Carl Sagan, who referred to arguments from ignorance as: ‘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….) This impatience with ambiguity can be criticized in the phrase: ‘absence of evidence is not evidence of absence.”

Sagan’s widow Ann Druyan stated that Sagan’s general position was that ‘science is saying in the absence of evidence, we must withhold judgment.’ In this, she also echoed the words of Bertrand Russell, who in 1959 said: ‘If you can’t find out whether [a thing] is true or whether it isn’t, you should suspend judgement.’ Astrophysicist Neil deGrasse Tyson expanded on UFOs, stating: ‘Remember what the U stands for (…): Unidentified. (…) If you don’t know what it is, then that’s where your conversation should stop, [rather] than say: ‘It must be…” anything. That’s what argument from ignorance is.’

Arguments from ignorance are often found along with false dilemmas (a situation in which limited alternatives are considered, when in fact there is at least one additional option). Arguments from ignorance have often been seen as a form of fallacy in informal logic that relies on ‘negative evidence’ and fail given that ignorance of an item should generally not lead to any conclusion about it, e.g. Bunnin and Yu state that: ‘Truth is one thing, and whether or not truth is known by us is another.’

Canadian author Douglas Walton states that despite the prevailing view that arguments from ignorance are generally fallacious, in some cases they lead to sound conclusions. Walton states that if we can assume that our ‘knowledge-base is complete,’ then an argument from ignorance can be nonfallacious; for anything not in a complete knowledge-base must be false.[ Arguments from ignorance, are however distinct from ‘arguments from silence’ used in fields such as historical analysis where the likelihood of a writer’s failure to mention an event is weighed.

A well known case of an argument from ignorance in legal proceedings was by senator Joseph McCarthy who stated: ‘I do not have much information on this except the general statement of the agency that there is nothing in the files to disprove his Communist connections.’ leading to the use of these arguments by his followers to accuse some people of being communists unless they could prove they were not.

Archeologist Lewis Binford has been generally critical of the use of ‘ad ignorantiam,’ and in particular ‘Ignoratio elenchi’ (‘irrelevant conclusion,’ the informal fallacy of presenting an argument that may or may not be logically valid, but fails nonetheless to address the issue in question); accusing forensic archaeologist Richard Gould of the ineffective use of these styles of argumentation. Archaeologist Clive Gamble points to Bronze Age pots found in the Shorncote quarry in Gloucestershire, England as an instance that absence of evidence is not always evidence of absence. Although no potter’s workshop or other evidence of local production facilities at the site can be found, the microscopic analysis of each potsherd’s fabric indicates that the pots were locally produced.

In evidence-based medicine, when a study is performed with a small sample size, the failure to detect a statistically significant relationship between two elements does not necessarily imply that none exists, and an absence of evidence can not be taken as an evidence of absence of an effect. In 1995, Altman and Bland pointed out that in medical research, errors in assuming that the absence of evidence may imply evidence of the absence of an effect continue to persists. In a 2004 editorial in the British Medical Journal Phil Alderson suggested the advice provided by Altman and Bland a decade before should be followed more stringently, and an absence of evidence in small studies should not be used to arrive at conclusions.

The US National Research Council report on environmental chemicals cautions that the failure to observe effects from the presence of chemicals in human tissue should not be used to draw the conclusion that they are not harmful, and that the distinction between absence of evidence effects and evidence of absence should be made clear. In some cases, an absence of evidence may be due to the lack of cooperation from a party subject to legal proceedings, e.g. in World Trade Organization (WTO) cases when a country accused of breaching trading rules specifically refuses to provide any evidence, WTO policies allow for inferences based on the refusal to cooperate.

Woods and Walton classify arguments from ignorance into three types: epistemic (cognitive), inductive (‘bottom-up’), and dialectic (logical). The epistemic type is presumptive in nature (presumed in the absence of further information) and attempts to manipulate the negation used in a proposition to shift the burden of knowledge via appeal to modal logic. The inductive type is often used in scientific reasoning and although it can at times produce false results, it may produce some valid results if used with caution (e.g. when sampling is performed and the sample size is not too small) yet it is by its nature usually inconclusive. The dialectic case is largely based on shifting the burden of proof on the other party and runs the risk of continuing as the parties repeatedly shift the burden between them.

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