Argumentum ad Ignorantiam:
Abstract: The argumentum
ad ignorantiam (the argument from ignorance or the appeal to
ignorance) is characterized with examples and shown to be sometimes
persuasive but normally fallacious.
Ad Ignorantiam (Argument from Ignorance of Appeal to Ignorance )Defined.
The Ad ignorantiam fallacy is the logical error
occurring when a proposition is unjustifiably claimed to be true simply
on the basis that it has not been proved false
the logical error occurring when a proposition is unjustifiably
claimed to be false simply because it has not been proved true.
This error in reasoning is often expressed with accompanying influential
The Ad Ignorantiam Argumentation
Scheme with Typical Examples.
The informal structure has two basic patterns:
Informal Guide to
Statement p is unproved
∴ Not-p is true.
Statement not-p is unproved
∴ p is true.
- Typical types of ad ignorantiam in the popular
media often include examples such as these:
- If one argues that God or telepathy, ghosts, or UFO's do not
exist because their existence has not been proved, then this fallacy
- On the other hand, if one argues that God, telepathy, and so on
do exist because their non-existence has not been proved,
then one argues fallaciously as well.
- Ad ignorantiam arguments can be either
of the fallacious or non-fallacious types.
- A straightforward, clear example of an ad
ignorantiam fallacy is shown in the following short argument
advanced by Weston Walch, a writer on railroad practices:
“It cannot be proved that government ownership of railroads
has succeeded in other countries. Therefore, government ownership
of the railroads should not be adopted in this country.”
From the fact that we do not know whether or not government ownership
of railroads is a success in other countries, we cannot logically
infer that government ownership would necessarily be unsuccessful in
- Often ad ignorantiam arguments in ordinary
language are not as simply stated as these examples provided here
— their basic informal scheme need to be analytically extracted
from the discourse under examination.
Non-Fallacious Ad Ignorantiam
Many writers on informal fallacies fail to point out that arguments fitting
the informal structure of the argumentum ad ignorantiam
can be non-fallacious.
- Non-fallacious ad ignorantiam arguments are often
cases where either …
(a) some evidence provides support for one of the opposing views
(b) one of the opposing views is contextually entrenched as a pragmatically
A typical example of an ad ignorantiam argument
which is not a fallacy is provided in the writings of the English
physician William Harvey, discoverer of the circulation of blood in human
beings. He argues in the following passage that the results of his
experiments should be considered credible in the absence of conclusive
“[W]hat I shall state I wish not to be taken as if i thought it a
voice from an oracle … I claim, however, that liberty … to
put forward as true such things as appear to be probable, until proved
to be manifestly false.”
If empirical evidence is available with no opposing evidence evident, then
it is reasonable to provisionally assume the position which has evidential
Typical Examples of the Ad Ignorantiam
- The following typical brief ad ignorantiam fallacy
examples are worth analyzing into the scheme discussed above. Note how
each of the arguments below is fallacious.
- In spite of all the talk, not a single governmental unidentified
aerial phenomenon report has been thoroughly authenticated. We may
assume, therefore, that UAPs do not exist.
- A biology professor consoles skittish students in lab before a
“There is no evidence that frogs actually feel pain; it is true
they exhibit pain behavior, but as they have no consciousness, they
feel no pain.
(But, of course, the premise is false; frogs have pain receptors and neural
pathways that indicate
Johnson: It is impractical to
send astronauts to Mars because the exorbitant expenses spent for such
a project should be spent on helping unfortunate persons.
Hanson: It is not at all impractical.
Johnson: What makes you think so?
Hanson: Well, try to prove me wrong.
(Hanson is defending his claim by an ad
ignorantiam, i.e., he inappropriately assumes his claim is true
if Johnson cannot refute it. Hanson is attempting to shift the burden of
proof to Johnson to provide evidence against the initial unsupported claim.
Historically, this example argument reflects the first
descriptions of the ad ignorantiam argument by the
philosopher John Locke; see the Postscript
- “Our universe, however, did begin with the primordial explosion,
since we can obtain no information about events that occurred before it.
The age of the universe, therefore, is the interval from the big bang
to the present.”
- The following typical brief ad ignorantiam
non-fallacious examples are worth analyzing into the scheme discussed
above. Note in each case how they are not usually considered
- Since no employee has filed objections to the provisional corporation
parking policies implemented during the last six months, the those policies
will be continued unless unforeseen problems arise.
“(AP)The Soviet news agency Tass declared
Saturday that the abominable snowman, thought by some to stalk the Himalayan
Mountains, does not exist.
Quoting arguments by Vadim Ranov, a man
described as a well-known Soviet explorer, Tass said that no remains —
skull or individual bones — had ever been found.
“Alleged yeti tracks spotted in the mountains
are more likely to be those of other animals distorted by bright sunrays,
Rhetorical and Persuasive Uses of the
Argumentum ad Ignorantiam
Rhetorical and persuasive uses of the Ad ignorantiam
are often similar to the technique of “raising doubts.”
E.g., suppose you wanted to convince a judge that you were not guilty
of driving too fast in a school zone by using this technique:
“Your honor, I'm sure you know how unreliable radar detectors are. Why,
I saw an a news program showing a radar detector timing a tree beside the
road at 50 mph, and in Florida, there was a case where the judge threw out
such evidence in a court proceeding. I certainly wasn't going that fast. Some
other car or truck could have sent back an erroneous signal. You might have
even timed a car passing me which looked like mine.”
The rhetorical technique of raising doubts is accomplished by asking
questions in an attempt to create uncertainty about an otherwise
Non-Fallacious Ad Ignorantiam
Arguments in Science and Law
In some specific situations, especially in scientific hypotheses and in the
law courts, one must, for practical reasons, assume that something is false
unless it is proved true or vice-versa.
E.g., in the legal system, “the assumption of innocence until
proved guilty” is a practical, not a strictly logical, process. Obviously,
initially someone can be found legally innocent at trial, but later found out
to be guilty of the crime.
- In many instances, if a practical decision must be made about
something that cannot be proved beyond doubt in spite of our serious attempts
to do so, then we are justified in presupposing,as a pragmatic consideration,
without deductive proof, the most reasonable alternative based upon the
available evidence, however slight.
At one time scientists concluded that DNA would not crystallize
because after testing, there was no proof that it would. This conclusion would
not be considered fallacious at the time even though now it is known that DNA
- Medical professor Theodore Woodward is often credited with the advice now
expressed in the aphorism taught in many medical schools:
“When you hear hoofbeats behind you, don't expect to see a
In other words, choose the most likely hypothesis for a circumstance in
need of explanation.
- Note how in the following argument having an ad ignorantiam
informal structure, no fallacy is present:
“Today we can be confident that a sample of uranium 238, no matter
what its origin, will gradually change into lead, and that this transmutation
will occur at a rate such that half of the uranium atoms will have become
lead in 4.5 billion years. There is no reason to believe that the nature
of rate of this process was any different in the very remote past, when the
universe was new.”
Note that this argument is structured as follows:
There is no evidence that the transmutation rate of U238 into
lead occurred any differently in the past.
In this example, assuming that no such change occurs as the universe ages makes
the science of radioactive decay and its uses possible.
∴ The transmutation rate did not change at a different rate
in the past.
- So the pragmatic application of the conclusion makes possible practical
- Also, whenever evidence is not available by which to choose among
hypotheses which must be made, applying
often provides a means by which to form a
The principle that entities are not to be multiplied without
necessity, sometimes also thought of as the principle of parsimony.
Of two hypotheses accounting for the same data, the simpler hypothesis
is to be adopted if neither hypothesis is supported by some other
Frequency of Use of Terms Ad
Ignorantiam, Argument from Ignorance, and Appeal to Ignorance
in Google Books 1700-2019
FIG. 1. Historical Frequency of Use of
“ad ignorantiam,” “argument from ignorance,”
and “appeal to ignorance” in Google Books 1700-2019
In the late 17th century, John Locke first described the ad
“Another way that Men ordinarily use to drive others, and force them
to submit their Judgments, and receive the Opinion in Debate, is to require
to Adversary to admit what they alledge as a Proof, or assign a better,
And this I call Argumentum ad
Notice that Locke is describing the ad ignorantiam
as a fallacy per se but as a argument for closure of a disagreement.
Dionysius Lardner in the mid-18th century interprets the import of Locke's
definition simply as “requiring your adversary to admit your principles,
or prove the contrary.”
Links to Additional Examples
and Online Practice Quizzes with Suggested Solutions for the Ad
Test your understanding of ad Ignorantiam arguments
appearing in the following quizzes:
Argumentum ad Ignorantiam
Fallacies of Relevance I
Fallacies of Relevance II
Fallacies of Relevance III
[Links go to page cited]