The argument from ignorance, also known as argumentum ad ignorantiam ("appeal to ignorance" ^[1]) or argument by lack of imagination, is a logical fallacy in which it is claimed that a premise is true only because it has not been proven false or is only false because it has not been proven true.

The argument from personal incredulity, also known as argument from personal belief or argument from personal conviction, refers to an assertion that because one personally finds a premise unlikely or unbelievable, the premise can be assumed not to be true, or alternately that another preferred but unproven premise is true instead.

Both arguments commonly share this structure: a person regards the lack of evidence for one view as constituting proof that another view is true. The types of fallacies discussed in this article should not be confused with the reductio ad absurdum method of argument, in which a valid logical contradiction of the form "A and not A" is used to disprove a premise.

Overview

Commonly in an Argument from Personal Incredulity or Argument from Ignorance, the speaker considers or asserts that something is false, implausible, or not obvious to them personally and attempts to use this gap in knowledge as "evidence" in favor of an alternative view of his or her choice. Examples of these fallacies are often found in statements of opinion which begin: "It is hard to see how...," "I cannot understand how...," or "it is obvious that..." (if "obvious" is being used to introduce a conclusion rather than specific evidence in support of a particular view).

Argument from ignorance

The two most common forms of the argument from ignorance, both fallacious, can be reduced to the following form:

• Something is currently unexplained or insufficiently understood or explained, so it is not (or must not be) true.
• Because there appears to be a lack of evidence for one hypothesis, another chosen hypothesis is therefore considered proven. The correction of this fallacy is often stated: "Absence of evidence is not evidence of absence."

Examples:

• "You can't prove God doesn't exist, so God exists"
• "You can't prove God does exist, so God doesn't exist"

Argument from personal incredulity

Two common versions of the argument from personal incredulity are:

• "I can't believe this is possible, so it can't be true." (The person is asserting that a proposition must be wrong because he or she is (or claims to be) unable or unwilling to fully consider that it might be true, or is unwilling to believe evidence which does not support her or his preferred view.)
• "That's not what people say about this; people instead agree with what I am saying." (Here the person is asserting that a proposition must be inaccurate because the opinion of "people in general" is claimed to agree with the speaker's opinion, without offering specific evidence in support of the alternative view.) This is also called argumentum ad populum.

An argument from personal incredulity is the same as an argument from ignorance only if the person making the argument has solely their particular personal belief in the impossibility of the one scenario as "evidence" that the alternative scenario is true (i.e., the person lacks relevant evidence specifically for the alternative scenario).

Quite commonly, the argument from personal incredulity is used in combination with some evidence in an attempt to sway opinion towards a preferred conclusion. Here too, it is a logical fallacy to the degree that the personal incredulity is offered as further "evidence." In such an instance, the person making the argument has inserted a personal bias in an attempt to strengthen the argument for acceptance of her or his preferred conclusion.

(Also see similar arguments: wisdom of repugnance and argument from emotion)

Burden of proof

An important aspect of the ad ignorantiam argument is establishing the burden of proof. While this concept is discussed in the law section of this page, it is important to realize that establishing the burden of proof is important in other arenas as well. All logic follows from presuppositions (axiomatic statements, see axiom). These presuppositions are not provable but are assumed as true.

Inductive usage

Inductive usage refers to the extension of an argument to support a wider generalization of a hypothesis, principle, scientific theory, or universal law. Many such uses of the Argument from Ignorance are considered fallacious, especially in academic papers which are expected to be rigorous about their key premises and empirical foundations. However, in some cases (such as that which the noted author Irving Copi describes below) where affirmative evidence could reasonably be expected to be found, but following careful unbiased examination, this evidence has still not been found, then it might become expedient, and sometimes even prudent, to infer that this might suggest (though it does not prove, deductively, it suggests inductively) that the evidence does not exist. Or, where the speaker can reasonably assume that all sane people will agree with a premise (e.g. "The sky is blue"), then he might decide it is unnecessary to provide evidence supporting that assertion; however, these issues (to which epistemological foundationalism is closely related, and with which it is also closely intertwined) are still debated.

Description

Irving Copi writes that:

The argumentum ad ignorantiam [fallacy] is committed whenever it is argued that a proposition is true simply on the basis that it has not been proven false, or that it is false because it has not been proven true. He adds, A qualification should be made at this point. In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence despite searching, as positive evidence towards its non-occurrence. (Copi 1953)

To support this, one might add a third case, the argument that something is false or true because the speaker cannot (or finds it hard to) conceive otherwise. This argument by lack of imagination is sometimes expressed in the form "Y is absurd (because I can not imagine it), therefore it must be untrue," or "It is hard to see how..." [ie I personally cannot see, or lack imagination, how], and is sometimes confused with the logically valid method of argument, reductio ad absurdum. A logical argument using reductio ad absurdum would be framed as "X logically leads to a provably impossible (absurd) conclusion, therefore it must be false." In reductio ad absurdum, it is necessary to show that accepting X implies a contradiction (such as "not X", or "Y and not Y" for some other proposition Y). In an argument from ignorance, the speaker asserts "X implies not Y", where Y is believed to be, but cannot be proven, true, rather than something which is provably contradictory.

Copi's argument concerns the Y condition; That in this case of "X implies not Y" for some other proposition Y, some weight must be given to the probability that the speaker's evaluation of Y is correct. For example, if proposition X is "This man was shot", and proposition Y is "There was no bullet", the speaker's qualification to assert condition Y must be considered. A coroner who had examined the body is most likely qualified to draw this conclusion, but an eyewitness is probably unqualified.

Argument from personal incredulity is very similar, e.g. "I am unable to believe/understand X, therefore it must be false."

Examples

• "The solar system must be younger than a million years because even if the sun were made of solid coal and oxygen it would have burned up within that time at the rate it generates heat." (An argument from ignorance, from 19th Century encyclopedias^{[citation needed]}, based on the assumption that because there was no means known at that time of producing heat more efficient than coal, this logically put a limit on the Sun's possible age. In fact in the 20th Century with the discovery of radioactivity and nuclear fusion, the sun's age was more correctly dated at many billions of years old instead. The 'ignorance' in this case was assuming that no fuel source could be more efficient than coal and oxygen.)

• "If polar bears are (the) dominant (predator) in the Arctic, then there would seem to have been no need for them to evolve a white-coloured form of camouflage." In his book Probability of God, Anglican Bishop Hugh Montefiore casts doubt on neo-Darwinian evolution with that statement. This argument was addressed by the evolutionary theorist Richard Dawkins in his book The Blind Watchmaker, who wrote that if the writer had thought to imagine a black polar bear trying to sneak up on a seal in the Arctic, he would see the evolutionary value of such fur. Moreover, the actually transparent fur is now known to be made up of small hollow tubes that are white, just like how clouds are white. The ignorance in this case was assuming that no other purpose could be served.

Law

In most modern criminal legal systems there is a presumption of innocence, and it is the responsibility of the prosecution to prove (usually "beyond reasonable doubt") that a defendant has in fact committed a particular crime. It is a logical fallacy to presume that mere lack of evidence of innocence of a crime is instead evidence of guilt. Similarly, mere lack of evidence of guilt cannot be taken as evidence of innocence. For this reason (among others), western legal systems err on the side of caution. Simply the act of taking a defendant before a court is not adequate evidence to presume anything. Courts require evidence of guilt to be presented first, adequate for the court to find that the charge has been substantiated-- i.e., that the prosecution's evidentiary burden has been met-- and only after this burden is met is the defense obliged to present counterevidence of innocence. If the burden of proof is not met, that does not imply that the defendant is innocent. Hence, in such a case, the defendant is found "not guilty", except in Scotland, where the jury also has the option to return a verdict of not proven.

Also, as a hypothetical example of an "argument from personal incredulity," defined above, suppose someone were to argue:

• I cannot imagine any way for Michael Jackson to have slept with young boys without having sex with them.
• Therefore he must be guilty of the crime of statutory rape.

Merely because the person making the argument cannot imagine how scenario "A" might have happened does not necessarily mean that the person's preferred conclusion (scenario "B") is correct. As with other forms of the argument from ignorance, the arguer in this instance has arrived at a conclusion without any evidence supporting the preferred hypothesis, merely for lack of being able to imagine the alternative.

The same principles of logic apply to the civil law, although the required burdens of proof generally are different. As well, these principles of logic apply to the introduction of a given component of a legal case by either a complainant or a defendant. That is, the mere lack of evidence in favor of a proposition put forth by a party in a legal proceeding (e.g., the assertion "she couldn't have left the house and returned in time to do X..." is offered without evidence in support) would not properly be taken as evidence in favor of an alternative explanation (e.g., "she did leave the house and return in time to do X...").

Science

Unexplained phenomena are an indication that a particular scientific theory does not provide a satisfactory model sufficient to explain or predict all outcomes. For example, the wave theory of light does not explain the photoelectric effect, though it successfully predicts the results of the double-slit experiment. However, later theories based around quantum mechanics provide an adequate explanatory model of both.

It is a logical mistake to assert that because a phenomenon is unpredictable by current scientific theories, that a better scientific theory cannot be found that provides an adequate natural explanatory model for the phenomena in question; and that therefore, one must assert that the only viable explanation of the unexplained phenomena is the supernatural action of God. This variant is known as the God-of-the-gaps argument.

However, it is also logically incorrect to assume that because a theory does explain all known relevant phenomena, it must be correct. The fact that no counter-examples are known to exist is not in itself proof of a given theory, since there is always the possibility of some yet-to-be-observed counter-example. For example, there are no known phenomena that are inconsistent with the Big Bang theory. This by no means constitutes definitive proof that the universe actually did originate with the Big Bang.

This is a criticism often levelled against the scientific community: scientists often do proceed as if a given theory - the Big Bang theory in this example - were definitively true. The defense is one of practicality: although in strict logical terms the assumption of correctness is fallacious, it is highly impractical (if not impossible) to devise experiments or evaluate data with absolutely no assumptions at all; science must have some set of base assumptions from which to operate.