Argument from ignorance

Argument from ignorance (from Latin: argumentum ad ignorantiam), also known as appeal to ignorance (in which ignorance represents "a lack of contrary evidence"), is a fallacy in informal logic. It asserts that a proposition is true because it has not yet been proved false (or vice versa). This represents a type of false dichotomy in that it excludes a third option, which is that: there may have been an insufficient investigation, and therefore there is insufficient information to prove the proposition be either true or false. Nor does it allow the admission that the choices may in fact not be two (true or false), but may be as many as four,

In debates, appeals to ignorance are sometimes used in an attempt to shift the burden of proof.

As described in Schreuder's Vision and Visual Perception:

Arguments that appeal to ignorance rely merely on the fact that the veracity of the proposition is not disproved 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. They do not inform upon reality. That is, whatever the reality is, it does not "wait" upon human logic or analysis to be formulated. Reality exists at all times, and it exists independently of what is in the mind of anyone. And the true thrust of science and rational analysis is to separate preconceived notion(s) of what reality is, and to be open at all times to the observation of nature as it behaves, so as truly to discover reality. 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 alternatives. A false dilemma may take the form:

Such arguments attempt to exploit the facts that (a) true things can never be disproved and (b) false things can never be proved. In other words, appeals to ignorance claim that the converse of these facts are also true. Therein lies the fallacy.

To reiterate, these arguments ignore the fact, and difficulty, that some true things may never be proved, and some false things may never be disproved with absolute certainty. The phrase "the absence of evidence is not the evidence of absence" can be used as a shorthand rebuttal to the second form of the ignorance fallacy (i.e. P has never been absolutely proved 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. In other words, where one researcher may say their experiment suggests evidence of absence, another researcher might argue that the experiment failed to detect a phenomenon for other reasons.

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