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Argumentum ad Ignorantiam:
The Argument
from Ignorance

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

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

   or

   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 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.

   or

   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 UFPs 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. Here are two examples of the fallacious type:
      
      
      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 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. For a more recent example, consider the following argument wherein news media outlets misreported the results of a meta-study on the effectiveness of wearing face masks during the Covid epidemic:

         The Washington Post reports “An author of the [Cochrane Review] study … declared of face masks in an interview, ‘There is just no evidence that they [i.e., masks] make any difference. Full stop.’”
         
         The New York Post concluded, “[A] major study from the Cochrane Library found that wearing them [masks] was pointless.”

         What the Cochrane meta-study found was that no good randomized controlled clinical trial for Covid disease had ever been conducted. Both newspapers quote the interview with Tom Jefferson, the lead author of the Cochrane study, as follows:

         “I keep saying it repeatedly, it [masking] needs to be looked at by doing a huge, randomized study — masks haven't been given a proper trial. They [i.e. randomized trials] should have been done, but they were not done.”

         And from the fact that no good study of masking has ever been conducted, no conclusion can be drawn.
         
         
         
      3. 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 does actually provide support for one of the opposing views
      

      or

      (b) one of the opposing views is contextually entrenched or assumed as a pragmatically useful presupposition in the argument being presented.
      
      
      
   2. 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 present, 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.
      
      
      
      1. Despite 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 description 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]
         
         (From the fact that physicists do not presently know whether or not anything existed before the Big Bang, we cannot logically conclude that nothing existed before that singularity. There may well be a prior universe or universes. For example, some string theory cosmologists provide a mathematical grounding for a cyclical universe extending infinitely in the past as well as the future.[5])
         
         
         
   2. The following typical brief ad ignorantiam non-fallacious examples are worth analyzing in accordance with the argument scheme discussed above. Note in each case how the arguments are not fallacious.
      
      
      
      1. Since no employee has filed objections to the provisional corporation parking policies implemented during the last six months, 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.”[6]

         
         
         

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 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 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 actually 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 was not 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.”[7]

      In other words, in some instances the most likely hypothesis can be rightfully chosen as a basis for making an unavoidable choice. Thus, when an uninformed decision must be made, we sometimes are pragmatically justified in making a probable presumption.
      
      
      
   3. For example, note how in the following argument having an ad ignorantiam informal structure, no fallacy is present even though a probable supposition is pragmatically adopted since its acceptance leads to practical consequences.

      “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.”[8]

      This argument can be 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.

      If we assume no such change in transmutation rate occurs in the future, the science of radioactive decay and its uses becomes 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
         close ×

         Occam's Razor

         The principle that entities are not to be multiplied without necessity, sometimes also thought of as the principle of parsimony.
         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

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

Postscript

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 the Adversary to admit what they alledge as a Proof, or assign a better, And this I call Argumentum ad Ignorantiam.”[9]

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.”[10]

Links to Additional Examples and Online Practice Quizzes with Suggested Solutions for the Ad Ignorantiam

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

Argumentum ad Ignorantiam Examples Exercise: Rather difficult examples of accident for analysis in a self-grading quiz.

Other Quizzes on Fallacies for Self-Testing:
Fallacies of Relevance I
Fallacies of Relevance II
Fallacies of Relevance III

Ad Ignorantiam Notes