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. This assertion is often encapsulated by the adage "absence of evidence is not evidence of absence."

Some uses of the argument by lack of imagination are considered fallacious. 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 because we don't have room for any train tracks. (the speaker fails to consider alternative forms of public transportation, such as buses)

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 could be falsified in various circumstances:

• When gift economies arise. (Of course, in the absence of intellectual property laws, to be considered a valid member of such an economy, an artist or inventor would have to give something other than an intellectual product, as it is impossible to "give" what one does not own in the first place; the creative, e.g. scientific and computer, "gift economies" that currently exist and have existed in the past are strongly based on the granting of license to use intellectual property and would therefore not be able to exist without intellectual property law.)
• When a work is commissioned for public display (such as an advertisement).

Also, in the extremely unlikely event that work would rarely be commissioned for display when it was discovered that such work, no longer covered by intellectual property laws, could be plagiarized with impunity, there would still be many other alternatives through which artists and inventors could make a living:

• Most artists and inventors are fully qualified to work as fast food service professionals, maintenance workers, or gravediggers.
• Female artists and inventors, simply by virtue of their sex, would usually be able to enter into the world's oldest profession.
• So long as drug prohibition exists, artists and inventors of either sex could get a job swallowing condoms filled with illegal drugs to get them past customs inspectors.
• Finally, it is even conceivable that without intellectual property laws, a form of "intellectual property black market" would evolve whereby mob bosses would demand percentages for the use of a given plagiarized idea. In such a case, an artist or inventor might be able to get a job enforcing the collection of such percentages.

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