by Tim Harding
The scope of the Appeal to Ignorance fallacy (Argumentum ad Ignorantiam in Latin) is more limited than its title would suggest. In the specific context of this fallacy, the word ignorance represents ‘a lack of contrary evidence’ rather than a lack of education or knowledge. The fallacy title was likely coined by the philosopher John Locke in the late 17th century.
In informal logic, this fallacy asserts that a proposition is true because it has not yet been shown to be false, or a proposition is false because it has not yet been shown to be true. This represents a type of false dichotomy, in that it excludes the possibility that there may have been an insufficient investigation to determine whether the proposition is either true or false. In other words, ‘absence of evidence is not evidence of absence.’
In rhetorical debates, appeals to ignorance are sometimes used in an attempt to shift the burden of proof. A typical example is as follows: ‘In spite of all the talk, not a single flying saucer report has been authenticated. We may assume, therefore, there are no such things as flying saucers.’ An absurd but logically equivalent example is: ‘Although NASA has shown that the surface of the moon is not made of green cheese, it has not conclusively demonstrated that the Moon’s core is not made of it; therefore, the moon’s core is made of green cheese.
This fallacy is a potential trap that empiricists need to be wary of falling into. We cannot prove the non-existence of anything, so the burden of proof lies with those who claim the existence of something, rather than those who doubt it. So, we should always remain open to the possibility of new evidence in support of a claim, even if no such evidence has ever been found.