Argument from ignorance

From Academic Kids

The argument from ignorance, also known as argumentum ad ignorantium or argument by lack of imagination, is the assertion that because something is currently inexplicable, it did not happen, or that because one cannot conceive of something, it cannot exist. This assertion is often summed up by the adage "absence of evidence is not evidence of absence."

Some uses of the argument by lack of imagination are considered fallacious. Irving Copi writes that:

The argumentum ad ignorantium [fallacy] is committed whenever it is argued that a proposition is true simply on the basis that it has not been proved false, or that it is false because it has not been proved true
A qualification should be made at this point. In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence as positive proof of its non-occurrence. (Copi 1953)

Argument by lack of imagination is sometimes expressed in the form "Y is absurd (because I can not imagine it), therefore it must be untrue." This is sometimes confused with the logically valid method of argument, reductio ad absurdum. A logical argument using reductio ad absurdum would be framed as "X logically leads to a provably impossible (absurd) conclusion, therefore it must be false." In reductio ad absurdum, it is necessary to show that X implies a contradiction (such as "not X", or "Y and not Y" for some other proposition Y). In an argument from ignorance, X implies something which the speaker considers absurd rather than something which the speaker can prove to be a contradiction.


  • I find it hard to imagine a way in which a thousand-ton piece of metal could fly through the air. Therefore, airplanes will never work.
  • This city can't handle public transportation because we don't have room for any train tracks. (the speaker fails to consider alternative forms of public transportation, such as buses, as well as the city's ability to appropriate land)


In most modern criminal legal systems, it is the responsibility of the prosecution to prove "beyond reasonable doubt" that the defendant is guilty. So in cases where the defendant has been acquitted, it is a logical fallacy to conclude that they were innocent - this would be to assume a proposition simply because it has not been proved false. The assumption of innocence is inspired by consideration for human rights, not by logical necessity.

As another example, suppose someone were to argue:

  • I cannot imagine any ways for artists and inventors to earn a living without intellectual property laws
  • Therefore such methods cannot exist
  • Therefore intellectual property laws are necessary

This would be an argument from lack of imagination, and can be shown false. In the absence of intellectual property, an artist or inventor does not usually sell their creation, they sell their service. Since anyone can copy their creation at whim, it is their unique services in creating new works, advertisements, live performances, or new inventions that are still valuable. Gift economies may also arise which support popular creators with the gifts of their admirers.


Unexplained phenomena are often an indication that a particular scientific theory is incomplete, or incorrect. For example, the wave theory of light does not explain the photoelectric effect, though it fits well with the results of the double-slit experiment. However, later theories based around quantum mechanics explain both. It would be a mistake to assert that because a phenomenon is unexplained by current scientific theories, it is unexplainable by science.

In his book Probability of God, Bishop Hugh Montefiore casts doubt on neo-Darwinian evolution with the following statement:

As for camouflage, this is not always easily explicable on neo-Darwinian premises. If polar bears are dominant in the Arctic, then there would seem to have been no need for them to evolve a white-coloured form of camouflage.

The corresponding (fallacious) argument against evolution can be presented as follows:

  • The evolutionary purpose served by camouflage is protection from predators.
  • Polar bears have no predators.
  • Therefore, camouflage serves no evolutionary purpose in polar bears.
  • Polar bears have camouflage.
  • Therefore, some attributes exist in nature which exhibit no evolutionary purpose.
  • Therefore, the theory of evolution does not account for some observable facts in nature.

The first assertion of this argument is an example of fallacy of the single cause: camouflage certainly could serve evolutionary purposes other than protection; for example, it might facilitate hunting wary prey. If one assumes that "protection from predators" is the only purpose for camouflage because one doesn't know of any other purpose, that is argument from ignorance.

As well, the fifth assertion of this argument is a subtle example of Illicit major, which is a formal syllogistic fallacy. It is true that polar bears evolved white fur, and it is true that white fur is camouflage; therefore, polar bears do have camouflage. However, if polar bears do not need camouflage, it would be fallacious to conclude that polar bears do not need white fur -- perhaps polar bears need white fur for reasons unrelated to camouflage. The fallacy is clearer if the argument is reworded:

  • White fur is camouflage.
  • Polar bears don't need camouflage.
  • Therefore, polar bears don't need white fur. (illicit major)

Observe that "don't need" could be replaced by "have yet don't need" to better correspond to the original argument. The three lines above were reworded for pedagogical clarity.


  • Irving M. Copi & Carl Cohen, Introduction to Logic. Prentice Hall College Div; 10th edition (1998). ISBN 0130102024.

See also

he:מן_הבורות uk:Argumentum ad Ignorantiam pt:Argumentum ad ignorantiam


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