Argument from Ignorance
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[edit] Definition
Argument from Ignorance (sometimes Argumentum ad Ignorantiam, from the Latin for "argument (aimed) at ignorance") is an informal logical fallacy where a participant draws a positive conclusion from a lack of contradictory evidence, frequently in an area where no such evidence can reasonably be expected. The most general structure of this argument runs something like the following:
- It cannot be shown that P is not true
- Therefore, P.
A simple variation on this is
- It cannot be shown that P is true
- Therefore, not-P.
This is a fallacy because the simple fact that no one knows how to contradict a statement does not mean that a statement is true. In areas such as science and technology, where new discoveries and inventions are always being made, new findings may arise at any time.
[edit] Examples
Example 1:
- Antagonist: No one has ever proven that the Yeti does not exist; therefore, I believe in the Yeti.
Example 2:
- Antagonist: No one has ever been able to crack RSA encryption, Therefore, RSA encryption must be secure.
Example 3:
- Antagonist: No one saw me anywhere near that window when it broke; therefore, it wasn't me that broke it.
Example 4:
- Antagonist (Senator Joe McCarthy): "I do not have much information on [case 40] except the general statement of the agency…that there is nothing in the files to disprove his Communist connections." [Source: Richard H. Rovere, Senator Joe McCarthy (Methuen, 1960), pp. 106-107.]
[edit] Exceptions to the Rule
In general, no inferences can be drawn from a lack of evidence. This is often summed up in epigrams such as "Absence of evidence is not evidence of absence." In some circumstances, though, the absence of evidence can be used as the base for an inference.
For example, we can usually believe that a railroad timetable is complete, which is to say that all departures and arrivals for a given location are listed. In this case, the following argument is valid (and not an appeal to ignorance).
- There is no arrival listed at this station in the next hour
- (suppressed premise) Any scheduled arrival would be listed
- Therefore, there is no scheduled arrival at this station in the next hour
In legal procedures, there is often an official burden of proof established under law. For example, the defendant may be "presumed innocent" unless the prosecution can prove his guilt. In this case, if the prosecutor cannot prove guilt, the defendant is considered to be proven innocent.
This "burden of proof" is often generalized informally in debate and dialogue, where the person presenting a claim -- particularly an unusual or improbable claim -- is usually considered to have the "burden of proof" in proving his claim. If he cannot provide evidence supporting his claim, many rational observers will reject his claim. This is particularly true where his claim contradicts well-established (and well-supported) aspects of reality such as the basic laws of physics. So the following argument is usually considered rational.
- Person X claims to have invented a perpetual motion machine
- Person X cannot (or will not) provide evidence that his perpetual motion machine works
- A working perpetual motion machine would violate the laws of thermodynamics
- Therefore, Person X is incorrect in his claim.
[edit] Similar Concepts
An argument from ignorance used to prove the existence of God is sometimes called a God of the Gaps Fallacy; the "gaps" being the incomplete understanding by science of the natural world. This type of argument is often found prefaced by the phrase "Science can't explain...". It should be remembered that 1) science has no pretensions about being able to explain everything, and 2) the progress of science is about discovering things that it was formerly unable to explain.
