Fallacy?

Argument from ignorance

What we don't yet know is treated as evidence for a conclusion.

Also called: Gaps argument · “Of the gaps” · Promissory note

The test

Is the conclusion supported by positive evidence, or only by what has not been found or explained?

This fallacy comes in two mirror-image forms, and it is only fair to name both. The first is the familiar “God of the gaps”: an unexplained phenomenon is credited to God simply because it is unexplained. The second is less often named: an unexplained phenomenon is credited to an unguided process because that is the only kind of explanation allowed, with a promise that the evidence will arrive later.

Absence of evidence can be evidence of absence, when we have looked where the evidence should be. Douglas Walton calls this a legitimate argument from negative evidence. The question is always how thorough the search has been.

Seen on both sides

In mainstream arguments
It would argue from ignorance to say that no one yet knows how a system arose, but it must have been an unguided process, and the details will be filled in later.
In critical arguments
It would argue from ignorance to say that science cannot explain a system, so it must have been designed, without positive evidence pointing to design.
Not a fallacy when

The absence is itself informative: a thorough search, where something should have been found, came up empty.

Diagnosed on this site

  • Claim № 2.02 · Cambrian explosionPossible
    In the claim as taught

    The ancestors are said to have been small, soft and unpreserved. The argument rests on what has not been found, with the molecular clock as the only positive evidence, and the clock depends heavily on its calibrations.

  • Claim № 4.03 · How rare are working proteins?Possible
    In the alternative reading

    “No undirected process has been observed to generate that much functional information” is an absence of evidence. It counts only if the search has been thorough enough that such a process should have been seen.

Further reading

  1. Walton DN. Arguments from Ignorance. Penn State University Press (1996).