PHIL102: Introduction to Critical Thinking and Logic

Argument from ignorance (argumentum ad ignorantiam), also known as appeal to ignorance (where ignorance represents "a lack of contrary evidence"), is a fallacy in informal logic. An argument from ignorance asserts that a proposition is true because it has not yet been proven false, or a proposition is false because it has not yet been proven true.

An argument from ignorance represents a type of false dichotomy because it excludes the possibility that there may have been an insufficient investigation to prove the proposition is true or false. It does not allow for the possibility that the answer is unknowable, only knowable in the future, or neither completely true nor completely false.

Debaters sometimes use appeals to ignorance to shift the burden of proof. In research, low-power experiments are subject to false negatives (there would have been an observable effect if there had been a larger sample size or better experimental design) and false positives (there was an observable effect, however this was a coincidence due purely to random chance, or the events correlate, but there is no cause-effect relationship). John Locke, the English philosopher, likely coined the term in the late 17th century.

Examples

False Positives

• "I wore red socks and we won the baseball game. My red socks helped win the game."
  
  
• "When ice cream sales increase, so do murders, therefore more ice cream causes more murders". (These events correlate due to the common element of high temperatures. High temperatures, not ice cream sales, leads to more murders)

Absence of Evidence

These examples contain or represent missing information:

• Statements that begin with "I can't prove it but..." are often referring to some kind of absence of evidence.
  
  
• "There is no evidence of aliens, and therefore, aliens do not exist" appeals to an absence of evidence.

Negative Results

These examples have the potential for "false negative" results:

• When the doctor says the test results were negative (a month later the test is positive).
  
  
• A patient stops using an antibiotic after only one day because they believe it is not working. The drug would have worked if they had used it for seven days.

Evidence of Absence

These examples contain definite evidence that can be used to show, indicate, suggest, infer or deduce the non-existence or non-presence of something:

• Someone carefully inspects the back seat of their car and finds no adult-sized kangaroos.
  
  
• The police did not find a gun in the suspect's clothing.
  
  
• The elderly patient did not have any teeth in his mouth.

Source: Wikipedia, https://en.wikipedia.org/wiki/Argument_from_ignorance
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