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Chapter 4
Lesson 8: The Argument From Ignorance
Do unicorns exist? Probably most of you would answer "no" to this question. In fact,
probably most of you would conclude that the opposite claim--"unicorns do not exist"--is
true. But how would you know that unicorns do not exist? What is your evidence? Most
of you probably would answer that neither have you ever seen a unicorn nor has anyone
ever provided any convincing evidence that proves the existence of unicorns. Of course,
you are correct; but does this really prove that unicorns don't exist? Have you looked
everywhere in the world for unicorns? Are there any isolated places on earth that have
not been well explored? Perhaps unicorns live there. In addition, it seems likely that
living organisms have originated and evolved on at least a few of the trillions of planets
in the universe (Hecht, 2002). Could there be unicorns on one of them? How would you
know?
The point in the previous paragraph is this: we can never prove that a claim is
universally false by simply failing to find evidence in support of it. This is because the
vastness of space and time limits the observations we can make. There is no way that we
can travel to all places and across all of time to make the necessary observations. This is a
"corollary" (a statement that follows logically from another statement) of the point made
in Lesson 7: we can never prove that a claim is universally true through verification.
As was stated there, although we become more confident that a claim is true as we
observe each new event consistent with the claim, we can never make all of the relevant
observations that would allow us to definitively state that the claim is true. In other
words, the claim can never be proved. The best we can do is to make so many
observations that we can be certain "beyond a reasonable doubt" that the claim is true. In
the case of "negative claims" (a claim that an event does not exist or does not occur),
there is a similar problem. Although we may conclude tentatively that a claim is false
after failing to find supporting evidence for it, the absence of supporting evidence can
never prove that the claim is false: we may one day find such supporting evidence.
The fact that we are unable to show conclusively that an event cannot occur or does not
exist is inappropriately used by those who make what has been called the "argument from
ignorance."
The Argument From Ignorance
Patrick smoked three packs of cigarettes per day for twenty years. Although he had tried
to quit before, he finally was successful only after he went to a hypnotherapist. Patrick's
best friend, Emad, questioned whether the hypnotherapy really helped Patrick to quit.
Perhaps, Emad speculated, it was simply a matter of "will power": perhaps Patrick was
more motivated to stop smoking this time. Patrick, however, responded by asking, "can
you prove that hypnotherapy didn't cause me to stop smoking?" When Emad answered,
"no," Patrick seemed satisfied, as if he had won the debate. Patrick's smug satisfaction
seemed to be due to an unstated conclusion he had made: Emad's lack of evidence for the
claim that hypnosis is unimportant for quitting smoking seemed to Patrick to mean that
hypnotherapy must have helped him to quit smoking.
In general, people who reason in this way are making the following argument (as stated
in Lesson 5, an argument is "a set of statements made up of reasons that support a
particular conclusion"):
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
No one has shown that A is impossible (where "A" is any particular claim, such as
the claim that "hypnosis cures addictive behavior" or that "unicorns exist").
Thus, A is possible.
If A is possible, then A is probable.
Thus, A is probable.
This is referred to as the "argument from ignorance." In essence, the argument from
ignorance (also called the "appeal-to-ignorance fallacy") states that a claim is likely to
be true because it hasn't been shown to be false (this is indicated in the third step of the
argument above). In other words, the argument from ignorance states that the lack of
definitive evidence against a claim is evidence in its favor (Engel, 2000; Jason, 2001).
For example, a person who argues that "there is no evidence that high-voltage electrical
lines do not cause cancer, therefore they probably do cause cancer" is making an
argument from ignorance. The argument goes something like this:
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


No one has provided convincing evidence that high-voltage electrical lines do not
cause cancer.
Thus, it is possible that high-voltage electrical lines cause cancer.
If it is possible that high-voltage electrical lines cause cancer, then it is probable
that high-voltage electrical lines cause cancer.
Thus, it is probable that high-voltage electrical lines cause cancer.
When each step of the argument is spelled out like this, it is easy to see why the argument
from ignorance is "fallacious." (A fallacious argument is one in which the reasons
given for a particular conclusion are false or irrelevant, or the conclusion does not
follow from the reasons presented.) To see why the argument from ignorance is
fallacious, you only have to extend what you learned in Lesson 5 regarding the
importance of evidence: a "claim's truth or falsity depends upon supporting or refuting
evidence to the claim, not the lack of support for a contrary or contradictory claim"
(Carroll, 2002).
Contexts in Which the Argument From Ignorance is Most Effective
Often, people making use of the argument from ignorance only imply a conclusion to the
argument. That is, they do not state the conclusion explicitly but, instead, allow their
audience to draw the conclusion. For example, a person who claims that ghosts exist may
ask an opponent, "can you prove that ghosts don't exist?" When the opponent mutters,
"no," the person beams triumphantly and waits for the audience to draw the conclusion
that the opponent's inability to provide conclusive evidence against the existence of
ghosts somehow is evidence for the claim that ghosts exist.
As this example suggests, it seems that the goal of people who use the argument from
ignorance is to place opponents on the defensive. That is, by asking, "can you prove that
A does not occur?" they are trying to shift the burden of proof to the opponent (who is
asking for evidence that A occurs). But the burden of proof should be on the person
who makes a claim, not on the opponent who questions the claim. For example, it
makes no sense to claim that megadoses of vitamins can cure most illnesses and then try
to support the claim by asking an opponent to prove that vitamins can't do this. Yet, in the
proper context, people sometimes can persuade others to accept their claim by using such
a tactic.
Which contexts work best for those making the argument from ignorance? It is easiest to
answer this question by looking at a couple of examples. First, it is unlikely that any of
you would be convinced by the following argument from ignorance (Adler, 1998):
"No one has shown that a gust of wind will not cushion my fall when I jump off a tall
building. Thus, it is probable that a gust of wind will cushion my fall when I jump off a
tall building."
On the other hand, at least some of you may find the following argument to be more
convincing:
"People have reported some pretty strange things involving unusual objects in the sky.
I've even seen some photographs of UFO's (unidentified flying objects) that experts have
a hard time explaining. And no one has shown that UFO's are not spacecraft from another
planet. Therefore, I think it is likely that at least some UFO's are alien spacecraft."
Can you identify where the argument from ignorance was used in this passage? It was
used in the following sentence: "And no one has shown that UFO's are not spacecraft
from another planet." Why is this use of the argument from ignorance more convincing
than it was in the passage about jumping off a tall building? It is more convincing
because the passage on UFO's also includes evidence that seems consistent with the claim
that UFO's are alien spacecraft ('people have reported strange experiences' and 'there are
photographs that are difficult to explain'). Nevertheless, this evidence, all by itself, is not
strong enough to support the conclusion that UFO's are alien spacecraft. Thus, the person
added on the argument from ignorance in order to shift the burden of proof to those who
question the claim: it is implied that opponents must first demonstrate that alien
spacecraft never visit earth. Thus, the argument from ignorance typically is used as an
"adjunct" (something that is added on in order to help something else) to an argument
that contains evidence insufficient to support a questionable claim.
Critical Thinking Questions
Question 8-1
Carefully examine the following statements:
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Kelly is pronounced "not guilty" because no one has disproved that she is
innocent.
Farah believes that we have immortal souls because no one has disproved the
existence of immortal souls.
Are both, one, or neither of these statements an example of the argument from ignorance?
Explain your answer. Suggested Answer
Question 8-2
Chapter 4 of the textbook discussed various processes of coping, which were defined as
"efforts to master, reduce, or tolerate the demands created by stress."
(a) How might the argument from ignorance be used as a way of coping with stress?
Suggested Answer
(b) Would this be an adaptive or a maladaptive way of coping? Explain your answer.
Suggested Answer
Question 8-3
Chapter 4 includes a discussion of the use of defense mechanisms as a means of coping
with stress. One such defense mechanism is repression. Sigmund Freud and other
psychodynamic theorists proposed that anxiety-inducing mental events (such as illicit
desires or memories of traumatic events) may be repressed--pushed into the unconscious-in order to reduce the anxiety that they cause. Various researchers (e.g., Holmes, 1990;
Loftus & Ketcham, 1994) have questioned the reality of repression. These critics have
even wondered if repression occurs at all.
(a) If you were to use the argument from ignorance to respond to these critics, what
would you say?
Suggested Answer
(b) What would be a more effective response to the critics of the concept of repression?
Suggested Answer
Question 8-4
Identify which of the following represents an argument from ignorance. Identify the
claim whose truth is inferred because no one has disproved it.
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People who use "hard drugs" such as heroin often begin by using marijuana and
alcohol. Thus, alcohol and marijuana eventually develop into hard drug use.
The dean stated that he had never heard of any drug problems on his campus. He
concluded that drug problems do not occur on his campus.
Four out of every five dentists recommend Beam toothpaste. If so many dentists
recommend Beam, you should use it.
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No one has ever accused the president of illegal activities. Therefore, he has never
done anything illegal.
I read in my psychology textbook that expressing anger reduces blood pressure.
They couldn't write that if it wasn't true. Therefore, it must be true.
The student received A's in all of her classes this semester. Thus, she must be very
intelligent.
I've never heard a "bad word" about him. Therefore, he must be a great guy!
Suggested Answer
Bibliography and References
Adler, J. E. (Jan/Feb, 1998) Open minds and the argument from ignorance. Skeptical
Inquirer, 22(1), 41-44. Retrieved April 6, 2002, from
http://www.csicop.org/si/9801/adler.html
Carrol, R. T. (2002, January 15). Argument to ignorance (argumentum ad ignorantiam).
Skeptic's Dictionary. Retrieved May 12, 2002, from http://skepdic.com/ignorance.html
Engel, S. M. (2000). With good reason: An introduction to informal fallacies (6th ed.).
Boston: Bedford/St. Martin's.
Hecht, J. (2002, May 15). Probability of alien life rises. New Scientist. Retrieved May 18,
2002, from http://www.newscientist.com/news/news.jsp?id=ns99992283
Holmes, D. S. (1990). The evidence for repression: An examination of sixty years of
research. In J. L. Singer (Ed.), Repression and dissociation: Implications for personality
theory, psychopathology, and health. Chicago: University of Chicago Press.
Jason, G. (2001). Critical thinking: Developing an effective worldview. Belmont, CA:
Wadsworth/Thomson Learning.
Loftus, E., & Ketcham, K. (1994). The myth of repressed memory: False memories and
allegations of sexual abuse. New York: St. Martin's Press.