Argument from ignorance

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.

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.

Examples:

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. We don't have room for any train tracks. (the speaker fails to consider alternative forms of public transportation, such as buses)

Table of contents

1 Law
2 Science
3 References

Law

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 would be falsified in various circumstances:

When a work is commissioned for public display (such as an advertisement).
When gift economies arise.

Science

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.

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 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, attributes exist in nature which exhibit no evolutionary purpose.
Therefore, the theory of evolution does not account for observable facts in nature.

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.

References

Irving Copi, I. M. (1953). Introduction to logic