Writing Tip #23: Types of Fallacies

Arguments can be logically invalid even though the conclusion may be true; in some cases, a conclusion can be true by coincidence. Furthermore, the statements (known as premises) that combine in a syllogism to yield the conclusion may not be true. For a syllogism to be conclusive, the premises must be true and the form must be valid. The resulting conclusion cannot be disputed. In most cases, however, our arguments are persuasive rather than conclusive because they leave room for further argument.

Nonetheless, arguments aim for conclusion. Remember that correlation is not cause and effect. For example, scientists recognize that smoking and lung cancer correlate positively at a high level, but scientists have yet to prove that smoking causes cancer. Thus, an argument could not conclude that smoking causes lung cancer, or conversely, that a person who does not smoke will not get lung cancer.

Below is a list of logical fallacies, which may be difficult to detect. A solid argument will not contain any of these fallacies.

  1. Appeal to symbols (I wrap myself in the flag to demonstrate my patriotism).
  2. Appeal to ignorance/unseen evidence (He's guilty because he had a gun we couldn't find).
  3. Appeal to illogical premises (It's OK to cheat, everybody does it in this class).
  4. Appeal to what's known (We know Germans like big cars, they make Mercedes).
  5. Red herring (She's rich. Did you hear her children own big houses and boats).
  6. Appeal to false authority (99% of family dentists say regular brushing is good; use Crest).
  7. Ad hominem (attacks character) (Of course you support euthanasia, your parents are dead).

includes name calling: My opponent is a liar.

includes prejudice: Women just can't be good stock brokers.

includes guilt by association: She's a feminist so her ideas must be radical.
  1. Straw person/straw man (You may think it's cheaper to cut trees to make sacks, but I don't).
  2. Begging the question (We should not give high grades because they will reward poor students as well as good ones).
  3. Complex questions (When did you stop speeding in your sports car?).
  4. Oversimplification (Love it or leave it!).
  5. Equivocation (He's so successful-successful had many meanings).
  6. Post hoc, ergo propter hoc (After this, therefore because of that) /Correlation

(I ate pizza and got a stomach ache, so pizza must not be good for us).
  1. Slippery slope (We don't dare provide more scholarships. We'll be supporting the whole country if we do).
  2. Generalization (The taxes are much too high).
includes pars pro toto (assumes what is true for part is true for all: That banker didn't pay his taxes on time. Bankers don't pay their taxes on time.) includes the opposite of pars pro toto (contradicting evidence is withheld: He was told to pay a penalty for late taxes. But, he paid his taxes on time; the mail was late).
  1. Faulty analogy (Well, the jet model worked in the wind tunnel, so that plane will fly).
  2. Non sequitur (Pete likes to drink milk, so Jennifer is sure to like malts ).
  3. Irrelevant reasons (I should not get below a B in this class because I worked hard).