FORMAL AND INFORMAL FALLACIES OF LOGIC
The formal fallacies of argumentation include:
1. Generalizing from insufficient data.
2. Using data that is biased (preselected to confirm an opinion already held) or not
randomly or representatively chosen.
3. Failing to support a claim with evidence.
4. Omitting a necessary part of the argument (for example, responding to objections or
neglecting a part of the subject).
The informal fallacies of argumentation include:
1. Faulty analogy. An analogy is a comparison of a subject with something more
familiar to the reader; its purpose is to clarify an idea. While it may clarify or reinforce a part of the argument, it cannot be used to prove a point. Faulty analogy is one
in which the two subjects that are compared have more differences than
similarities. Comparing the community college to an aircraft carrier (because both allow
you to "take off") is indulging in a faulty analogy. If you use an analogy, ask yourself if the comparison is natural—if the things compared share enough similarities to
be able to support each other. If the analogy is far-fetched or trivial, delete it.
2. Non Sequitur. "It does not follow." Your conclusion does not follow from the evidence presented. Consider the argument: "he's certainly sincere; (therefore) he must
be right." Sincere people have been wrong many times in history; it does not follow
that he is right because he is sincere.
3. Post Hoc. This is also known as ergo propter hoc or false cause. This is a simplistic
cause-and-effect argument: because that is true, so is this,as in "drive our car, and the
girls (or boys) will swarm all over you." There is no logical connection between driving the car and the fact of sex appeal. (Of course, if enough people accept the faulty
claim, it may be borne out in experience). At times, post hoc arguments may seem
quite reasonable when in fact they oversimplify complex subjects. Take, for example,
the claim that "the stock market rose because of the president's tax policies." The president's tax policies may be one of many factors affecting the rise or fall of the stock
market; a conclusion such as the above ignores this fact, and draws too close of a connection between this (tax policies) and that (stock market rise).
4. Begging the Question. This argument assumes in its premises the things you should
be establishing by proof. It is also known as "loading the assertion," and perhaps it
most commonly appears as labelling. When someone argues that "these filthy books
should not be allowed in our library" without defining what "filth" is and demonstrating how these books are "filthy," he or she is evading the burden of proof by begging
the question. Caution: statements such as "it's obvious that. . ." or "everybody knows.
. ." make the critical reader aware that the writer may be begging the question; be careful
of making such statements unless you know absolutely that everybody does know what
you claim.
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5. Ad hominem. Ad hominem, or arguments "to the person," focus on the individual's
character or personal life rather than on the issues involved. When Jack Kennedy ran
for president, many of those who opposed him tried to divert the debate from the issues facing the nation to the fact that he was a catholic. At times, of course, such
issues can become legitimate concerns—a six-time bank robber should not be elected
city treasurer, for example—but one should generally try to distinguish between personal attacks and attempts to explore the substance of an issue.
6. Tu quoque. "You're another": these arguments evade discussion of an issue by making a hostile charge against those one disagrees with. This argument often is a form of
ad hominem, attacking an opponent's character or indulging in name-calling rather
than examining evidence.
7. Ad populum. The argument "to the people" is also a variation on ad hominem, appealing to popular biases (such as God, motherhood, one's nation, family, or negative
biases such as communism, nazism, etc.). These arguments seek to arouse the emotions rather than examine evidence.
8. Either-Or. This is an attempt to force one's opponent to choose between two conflicting options, assuming one is either for one or the other, when in fact there may be
a whole variety of options available. During the Vietnam era, those on the right who
claimed one should either "love it (America) or leave it," and those on the left who
claimed one should either hate the cops or be a "fascist pig" were both guilty of the
either-or fallacy.
9. The Complex Question. Such a question is phrased so that a direct answer can only
support the embedded assumption. "When did you start cheating on your girlfriend?"
is a complex question when the questioner has not already proven that such cheating
occurred. Complex questions should be divided into a series of questions so that each
assumption can be examined thoroughly and shown to be correct before proceeding to
larger questions or conclusions.
10. Straw man or red herring. This is a digression used to sidetrack the argument. For
example, the anti-environmentalist who tries to discredit concerns about Americangenerated air pollution by referring to air pollution generated by communist countries
or to the eruption of Mt. Pinatubo is evading the issue by setting up a straw man and
attacking that instead of facing his or her opponent's claims fairly. Often issues used
to sidetrack the argument may be worth discussing, but they do not pertain to the evidence and conclusions regarding the stated issue.
11. The genetic fallacy. This is an assertion that we can predict the nature and character
of something or someone by knowing its/his or her origins. Its most common form is
seen in racial or ethnic prejudice, and in stereotypes of all kinds.
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12. Special Pleading. This is the same thing as the formal fallacy of using biased data
or only sing data that supports a pre-selected claim. Bertrand Russell's Why I Am Not
a Christian exhibits a case of special pleading; Russell examines only the bad things
Christians have done in order to support his conclusion that this religion is evil.
13. Appeal to ignorance. This is the assertion that one's conclusion must be right because no one has proven it wrong. The burden of proof is on the one who makes the
claim, and the appeal to ignorance is really an attempt to evade the responsibility to
present supporting evidence for one's contentions.