Argument from ignorance, also known as appeal to ignorance, is a fallacy in informal logic. It 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. This represents a type of false dichotomy in that it excludes the possibility that there may have been an insufficient investigation to prove that the proposition is either true or false. It also does not allow for the possibility that the answer is unknowable, only knowable in the future, or neither completely true nor completely false. In debates, appeals to ignorance are sometimes used in an attempt to shift the burden of proof. In research, low-power experiments are subject to false negatives and false positives. The term was likely coined by philosopher John Locke in the late 17th century.
"There is no evidence of aliens, and therefore, aliens do not exist" appeals to an absence of evidence.
"A recent study said there is no strong evidence that shows flossing reduces cavities or gum disease." The NIH dental health expert indicated that large-scale, long-term clinical trials are expensive and challenging to perform, and that patients would still likely benefit from flossing.
False negatives
These examples have the potential for "false negative" results.
When the doctor says that the test results were negative.
Under "Termites" the inspector checked the box that read "no".
A patient uses an antibiotic for only one day and stops because they feel it isn't working..
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.
One very carefully inspects the back seat of one's 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.
Arguments from ignorance
"I take the view that this lack is the most ominous sign in our whole situation. It convinces me more than perhaps any other factor that the sabotage we are to get, the Fifth Column activities are to get, are timed just like Pearl Harbor ... I believe we are just being lulled into a false sense of security." – Earl Warren, then California's Attorney General.
This example clearly states what appeal to ignorance is: "Although we have proven that the moon is not made of spare ribs, we have not proven that its core cannot be filled with them; therefore, the moon’s core is filled with spare ribs."
Carl Sagan explains in his book The Demon-Haunted World:
Related terms
Contraposition and transposition
is a logically validrule of inference that allows the creation of a new proposition from the negation and reordering of an existing one. The method applies to any proposition of the type If A then B and says that negating all the variables and switching them back to front leads to a new proposition i.e. If Not-B then Not-A that is just as true as the original one and that the first implies the second and the second implies the first. Transposition is exactly the same thing as Contraposition, described in a different language.
Null result
Null result is a term often used in science to indicate evidence of absence. A search for water on the ground may yield a null result ; therefore, it probably did not rain.
Related arguments
Argument from self-knowing
Arguments from self-knowing take the form:
If P were true then I would know it; in fact I do not know it; therefore P cannot be true.
If Q were false then I would know it; in fact I do not know it; therefore Q cannot be false.
In practice these arguments are often unsound and rely on the truth of the supporting premise. For example, the claim that If I had just sat on a wild porcupine then I would know it is probably not fallacious and depends entirely on the truth of the first premise.
