What Is an Appeal to Ignorance (Fallacy)?

The appeal to ignorance is a fallacy based on the assumption that a statement must be true if it cannot be proven false — or false if it cannot be proven true. Also known as argumentum ad ignorantiam and the argument from ignorance.

The term argumentum ad ignorantiam was introduced by John Locke in his "Essay Concerning Human Understanding" in 1690. 

Examples

Appeal to ignorance fallacy examples can include abstractions, the physically impossible to prove, and the supernatural. For example, someone says that there's life in the universe because it hasn't been proven to not exist outside of our solar system or that UFOs have visited Earth. Perhaps a person postulates that every action human beings take is fated because no one has proven that people have free will. Or maybe someone says that ghosts exist because you can't prove that they don't; all of these are appeals to ignorance fallacies. 

"One interesting aspect of the appeal to ignorance is that the same appeal can be used to support two conclusions that are diametrically opposed to each other. This paradox is a telltale clue that appeals to ignorance involve flawed reasoning. It is easy to see what is wrong with appeals to ignorance when the opposite arguments (ghosts exist — ghosts do not exist) are presented together and the lack of evidence on the issue under discussion is obvious. However, when the same fallacy surfaces in more complex debates and the appeal to ignorance is not as blatant, the strategy can be more difficult to recognize."

Examples can be more mundane as well, such as the belief that a policy or law is good and working well just because no one has yet objected to it or the belief that every student in a class understands the material fully because no one has raised a hand to ask a question of the professor.

How They're Manipulated

People can use this fallacy to manipulate others because there is often an appeal to people's emotions within the proposed ideas. The assertion then puts nonbelievers in the fallacy on the defensive, which is irrational, as the person proposing the idea should have the burden of proof, wrote S. Morris Engel, in the third edition of "With Good Reason".

Howard Kahane and Nancy Cavender, authors of "Logic and Contemporary Rhetoric," gave the example of Senator Joseph McCarthy, who accused a whole list of people of being communist without proof, severely damaging their reputations just because of the accusations:

"In 1950, when Senator Joseph R. McCarthy (Republican, Wisconsin), was asked about the fortieth name on a list of 81 names of people he claimed were communists working for the United States Department of State, he responded that '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.'
"Many of McCarthy's followers took this absence of evidence as proof that the person in question was indeed a communist, a good example of the fallacy of appeal to ignorance. This example also illustrates the importance of not being taken in by this fallacy. No scrap of relevant evidence ever was presented against any of the people charged by Senator McCarthy, yet for several years he enjoyed great popularity and power; his 'witch hunt' ruined many innocent lives." (10th ed. Thomson Wadsworth, 2006)

In a Courtroom

The appeal to ignorance is generally not fallacious in a criminal court where an accused person is presumed innocent until proven guilty. The prosecution has to present enough evidence to convict someone — proof that goes beyond a reasonable doubt — or else the person goes free. "Thus argument from ignorance is fundamental to the argumentation structure of the trial in the adversary system."

Combatting the Fallacy

Though it's good to keep an open mind in case evidence for an assertion comes to light, critical thinking will be what comes to your aid when examining an appeal to ignorance. Think of what Galileo went through when he postulated about the solar system or other scientific or medical breakthroughs that have come to light in recent decades if not centuries — an existing theory was challenged by proof and then eventually changed. But a change in longheld beliefs doesn't come easily, and some things are just impossible to test (life in the universe, and the existence of God).  

Sources

• Wayne Weiten, "Psychology: Themes and Variations, Briefer Version," 9th ed. Wadsworth, Cengage, 2014
• Douglas Walton, "Methods of Argumentation." Cambridge University Press, 2013