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MS Douce 308 Li Romans de Cassamus 
	fol. 4r; Bodlelan Library, detail

Argumentum ad Ignorantiam:
The Argument
from Ignorance

Abstract: The argumentum ad ignorantiam (the argument from ignorance or the appeal to ignorance) is characterized with examples and shown to be sometimes persuasive but normally fallacious.

  1. The Argumentum Ad Ignorantiam (Argument from Ignorance of Appeal to Ignorance )Defined.

    The Ad ignorantiam fallacy is the logical error occurring when a proposition is unjustifiably claimed to be true simply on the basis that it has not been proved false


    the logical error occurring when a proposition is unjustifiably claimed to be false simply because it has not been proved true.

    This error in reasoning is often expressed with accompanying influential rhetoric.

  2. The Ad Ignorantiam Argumentation Scheme with Typical Examples.

    The informal structure has two basic patterns:

    Informal Guide to ad Ignorantiam

    Statement p is unproved

    Not-p is true.


    Statement not-p is unproved

    p is true.

    1. Typical types of ad ignorantiam in the popular media often include examples such as these:

      1. If one argues that God or telepathy, ghosts, or UFO's do not exist because their existence has not been proved, then this fallacy occurs.

      2. On the other hand, if one argues that God, telepathy, and so on do exist because their non-existence has not been proved, then one argues fallaciously as well.

    2. Ad ignorantiam arguments can be either of the fallacious or non-fallacious types.

      1. A straightforward, clear example of an ad ignorantiam fallacy is shown in the following short argument advanced by Weston Walch, a writer on railroad practices:
        “It cannot be proved that government ownership of railroads has succeeded in other countries. Therefore, government ownership of the railroads should not be adopted in this country.”[1]
        From the fact that we do not know whether or not government ownership of railroads is a success in other countries, we cannot logically infer that government ownership would necessarily be unsuccessful in this country.

      2. Often ad ignorantiam arguments in ordinary language are not as simply stated as these examples provided here — their basic informal scheme need to be analytically extracted from the discourse under examination.

  3. Non-Fallacious Ad Ignorantiam Arguments

    Many writers on informal fallacies fail to point out that arguments fitting the informal structure of the argumentum ad ignorantiam can be non-fallacious.

    1. Non-fallacious ad ignorantiam arguments are often cases where either …

      (a) some evidence provides support for one of the opposing views


      (b) one of the opposing views is contextually entrenched as a pragmatically useful presupposition.

    2. “William Harvey” (oil on canvas; detail) 
		Daniel Mijtens (attrib.) 
		1627  National Portrait Gallery, London NPG 5115 A typical example of an ad ignorantiam argument which is not a fallacy is provided in the writings of the English physician William Harvey, discoverer of the circulation of blood in human beings. He argues in the following passage that the results of his experiments should be considered credible in the absence of conclusive countervailing evidence:
      “[W]hat I shall state I wish not to be taken as if i thought it a voice from an oracle … I claim, however, that liberty … to put forward as true such things as appear to be probable, until proved to be manifestly false.”[2]
      If empirical evidence is available with no opposing evidence evident, then it is reasonable to provisionally assume the position which has evidential support.

  4. Typical Examples of the Ad Ignorantiam Arguments

    1. The following typical brief ad ignorantiam fallacy examples are worth analyzing into the scheme discussed above. Note how each of the arguments below is fallacious.

        “USS Nimitz ‘Tic Tac’ Event”
			Commander David Fravor; Unidentified Aerial Phenomenon 
			FBI/Navy FOIA Release
      1. In spite of all the talk, not a single governmental unidentified aerial phenomenon report has been thoroughly authenticated. We may assume, therefore, that UAPs do not exist.

      2. A biology professor consoles skittish students in lab before a dissection:
        “There is no evidence that frogs actually feel pain; it is true they exhibit pain behavior, but as they have no consciousness, they feel no pain.
        (But, of course, the premise is false; frogs have pain receptors and neural pathways that indicate sensation.)[3]

      3. Johnson: It is impractical to send astronauts to Mars because the exorbitant expenses spent for such a project should be spent on helping unfortunate persons.

        Hanson: It is not at all impractical.

        Johnson: What makes you think so?

        Hanson: Well, try to prove me wrong.

        (Hanson is defending his claim by an ad ignorantiam, i.e., he inappropriately assumes his claim is true if Johnson cannot refute it. Hanson is attempting to shift the burden of proof to Johnson to provide evidence against the initial unsupported claim.

        Historically, this example argument reflects the first descriptions of the ad ignorantiam argument by the philosopher John Locke; see the Postscript below.)

      4. “Our universe, however, did begin with the primordial explosion, since we can obtain no information about events that occurred before it. The age of the universe, therefore, is the interval from the big bang to the present.”[4]

    2. The following typical brief ad ignorantiam non-fallacious examples are worth analyzing into the scheme discussed above. Note in each case how they are not usually considered fallacious.

      1. Since no employee has filed objections to the provisional corporation parking policies implemented during the last six months, the those policies will be continued unless unforeseen problems arise.

      2. “(AP)The Soviet news agency Tass declared Saturday that the abominable snowman, thought by some to stalk the Himalayan Mountains, does not exist.

        Quoting arguments by Vadim Ranov, a man described as a well-known Soviet explorer, Tass said that no remains — skull or individual bones — had ever been found.

        “Alleged yeti tracks spotted in the mountains are more likely to be those of other animals distorted by bright sunrays, Tass said.[5]

  5. Rhetorical and Persuasive Uses of the Argumentum ad Ignorantiam

    Rhetorical and persuasive uses of the Ad ignorantiam are often similar to the technique of “raising doubts.”

    E.g., suppose you wanted to convince a judge that you were not guilty of driving too fast in a school zone by using this technique:
    “Your honor, I'm sure you know how unreliable radar detectors are. Why, I saw an a news program showing a radar detector timing a tree beside the road at 50 mph, and in Florida, there was a case where the judge threw out such evidence in a court proceeding. I certainly wasn't going that fast. Some other car or truck could have sent back an erroneous signal. You might have even timed a car passing me which looked like mine.”
    The rhetorical technique of raising doubts is accomplished by asking questions in an attempt to create uncertainty about an otherwise settled question.

  6. Non-Fallacious Ad Ignorantiam Arguments in Science and Law

    In some specific situations, especially in scientific hypotheses and in the law courts, one must, for practical reasons, assume that something is false unless it is proved true or vice-versa.

    E.g., in the legal system, “the assumption of innocence until proved guilty” is a practical, not a strictly logical, process. Obviously, initially someone can be found legally innocent at trial, but later found out to be guilty of the crime.

    1. In many instances, if a practical decision must be made about something that cannot be proved beyond doubt in spite of our serious attempts to do so, then we are justified in presupposing,as a pragmatic consideration, without deductive proof, the most reasonable alternative based upon the available evidence, however slight.

      At one time scientists concluded that DNA would not crystallize because after testing, there was no proof that it would. This conclusion would not be considered fallacious at the time even though now it is known that DNA will crystallize.

    2. Medical professor Theodore Woodward is often credited with the advice now expressed in the aphorism taught in many medical schools:
      “When you hear hoofbeats behind you, don't expect to see a zebra.”[6]
      In other words, choose the most likely hypothesis for a circumstance in need of explanation.

    3. Note how in the following argument having an ad ignorantiam informal structure, no fallacy is present:
      “Today we can be confident that a sample of uranium 238, no matter what its origin, will gradually change into lead, and that this transmutation will occur at a rate such that half of the uranium atoms will have become lead in 4.5 billion years. There is no reason to believe that the nature of rate of this process was any different in the very remote past, when the universe was new.”[7]
      Note that this argument is structured as follows:
      There is no evidence that the transmutation rate of U238 into lead occurred any differently in the past.
      The transmutation rate did not change at a different rate in the past.
      In this example, assuming that no such change occurs as the universe ages makes the science of radioactive decay and its uses possible.

      1. So the pragmatic application of the conclusion makes possible practical benefits.

      2. Also, whenever evidence is not available by which to choose among hypotheses which must be made, applying Occam's Razor often provides a means by which to form a supposition.

        Of two hypotheses accounting for the same data, the simpler hypothesis is to be adopted if neither hypothesis is supported by some other hypothesis.

  7. Frequency of Use of Terms Ad Ignorantiam, Argument from Ignorance, and Appeal to Ignorance in Google Books 1700-2019

    Ngram graph showing historical frequency of ad 
	ignorantiam, argument from ignorance, and appeal to ignorance in Google books

    FIG. 1. Historical Frequency of Use of “ad ignorantiam,” “argument from ignorance,” and “appeal to ignorance” in Google Books 1700-2019


In the late 17th century, John Locke first described the ad ignorantiam argument:
“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 to Adversary to admit what they alledge as a Proof, or assign a better, And this I call Argumentum ad Ignorantiam.”[8]
Notice that Locke is describing the ad ignorantiam, not as a fallacy per se but as a argument for closure of a disagreement.

Dionysius Lardner in the mid-18th century interprets the import of Locke's definition simply as “requiring your adversary to admit your principles, or prove the contrary.”[9]

Test your understanding of ad Ignorantiam arguments appearing in the following quizzes:

Argumentum ad Ignorantiam Examples Exercise
Fallacies of Relevance I
Fallacies of Relevance II
Fallacies of Relevance III

Ad Ignorantiam Notes

[Links go to page cited]

1. John Weston Walch, Complete Handbook on Government Ownership or Railroads (Platform News, 1939), 138.

2. William Harvey, “On Conception,” The Works of William Harvey, M.D. trans. Robert Willis (London: Sydenham Society, 1847), 575.

3. Sarah Annie Guénette, Marie-Chantal Giroux, and Pascal Vachon, “Pain Perception and Anaesthesia in Research Frogs, Experimental Animals 62 no. 2 (2013), 87-92. doi: 10.1538/expanim.62.87

4. David Schramm, “The Age of the Elements,” Scientific American) 230 no. 1 (January, 1974), 70.

5. “Abominable Snowman Doesn't Exist,” Greenville News 110 no. 99 (April 8, 1984), 11.

6. Andrew Holtz, The Medical Science of House, M.D. (New York: Berkeley Publishing, 2006), 27.

7. David Schramm, “The Age of the Elements,” 67.

8. John Locke, An Essay Concerning Humane Understanding in Four Books 3rd ed. (1689 London: Awnsham, John Churchil, and Samuel Manship 1695), 306.

9. Dionysius Lardner, Lectures Upon Locke's Essay (Dublin: Hodges and Smith, 1831), 160.

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