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Argument from ignorance

The argument from ignorance (ad ignorantium), or argument by lack of imagination, is that because something is currently inexplicable, it did not happen, or that because one cannot conceive of something, it cannot exist.

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 propostion 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.

Table of contents
1 Law
2 Science
3 References


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. That the law requires a person to be assumed innocent if not proven guilty is inspired by consideration for human rights, not by logical necessity.

As another example, suppose someone was to argue:

This would be an argument from lack of imagination, and would be falsified in various circumstances:


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 wave-particle duality explain both. It would be a mistake to assert that because a phenomenon is unexplained by current scientific theories, it is unexplainable by science.

Richard Dawkins has attributed an equivalent of the following argument to Bishop Hugh Montefiore, referring to the argument from ignorance as the "argument from personal incredulity".

The first assertion above turns this into an argument from ignorance. In fact polar bears benefit from the fact that their camouflage conceals them from their prey. Protection from predators is only one benefit of camouflage - the argument above fails to acknowledge that there could be other benefits, and hence is fallacious.