Rodin's Thinker in a Dunce Cap

Appeal to Ignorance

Alias:
  • Argument from Ignorance
  • Argumentum ad Ignorantiam
Type: Informal Fallacy
Forms
There is no evidence against p.
Therefore, p.		 There is no evidence for p.
Therefore, not-p.

Example:

[Joe McCarthy] announced that he had penetrated "Truman's iron curtain of secrecy" and that he proposed forthwith to present 81 cases… Cases of exactly what? "I am only giving the Senate," he said, "cases in which it is clear there is a definite Communist connection…persons whom I consider to be Communists in the State Department." … Of Case 40, he said, "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."

Source: Richard H. Rovere, Senator Joe McCarthy (Methuen, 1960), pp. 106-107. Cited in: Irving M. Copi, Introduction to Logic (Fourth Edition) (1972), p. 88.

Exposition:

An appeal to ignorance is an argument for or against a proposition on the basis of a lack of evidence against or for it. If there is positive evidence for the conclusion, then of course we have other reasons for accepting it, but a lack of evidence by itself is no evidence.

Exposure:

There are a few types of reasoning which resemble the fallacy of Appeal to Ignorance, and need to be distinguished from it:

  1. Sometimes it is reasonable to argue from a lack of evidence for a proposition to the falsity of that proposition, when there is a presumption that the proposition is false. For instance, in American criminal law there is a presumption of innocence, which means that the burden of proof is on the prosecution, and if the prosecution fails to provide evidence of guilt then the jury must conclude that the defendant is innocent. Similarly, the burden of proof is usually on a person making a new or improbable claim, and the presumption may be that such a claim is false. For instance, suppose that I claim that I was taken by flying saucer to another planet, but when challenged I can supply no evidence of this unusual trip. It would not be an Appeal to Ignorance for you to reason that, since there is no evidence that I visited another planet, therefore I probably didn't do so.
  2. We sometimes have meta-knowledge—that is, knowledge about knowledge—which can justify inferring a conclusion based upon a lack of evidence. For instance, schedules—such as those for buses, trains, and airplanes—list times and locations of arrivals and departures. Such schedules usually do not attempt to list the times and locations when vehicles do not arrive or depart, since this would be highly inefficient. Instead, there is an implicit, understood assumption that such a schedule is complete, that all available vehicle departures and arrivals have been listed. Thus, we can reason using the following sort of enthymeme:

    There is no departure/arrival listed in schedule S for location L at time T.
    Suppressed Premiss: All departures and arrivals are listed in schedule S.
    Therefore, there is no departure/arrival for location L at time T.

    This kind of completeness of information assumption is often called the "closed world assumption". When it is reasonable to accept this assumption—as with plane or bus schedules—it is not a fallacy of appeal to ignorance to reason this way.

  3. Another type of reasoning is called "auto-epistemic" ("self-knowing") because it involves reasoning from premisses about what one knows and what one would know if something were true. The form of such reasoning is:

    If p were true, then I would know that p.
    I don't know that p.
    Therefore, p is false.

    For instance, one might reason:

    If I were adopted, then I would know about it by now.
    I don't know that I'm adopted.
    Therefore, I wasn't adopted.

    Similarly, when extensive investigation has been undertaken, it is often reasonable to infer that something is false based upon a lack of positive evidence for it. For instance, if a drug has been subjected to lengthy testing for harmful effects and none has been discovered, it is then reasonable to conclude that it is safe. Another example is:

    If there really were a large and unusual type of animal in Loch Ness, then we would have undeniable evidence of it by now.
    We don't have undeniable evidence of a large, unfamiliar animal in Loch Ness.
    Therefore, there is no such animal.

    As with reasoning using the closed world assumption, auto-epistemic reasoning does not commit the fallacy of Argument from Ignorance.

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