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Unwarranted Assumptions: Fallacies

The fallacy of unwarranted assumption is committed when the conclusion of an argument is based on a premise (implicit or explicit) that is false or unwarranted. An assumption is unwarranted when it is false—these premises are usually suppressed or vaguely written. An assumption is also unwarranted when it is true but does not apply in the given context—these premises are usually explicit. The varieties of unwarranted assumptions are too numerous to classify, but a few examples should give you the basic idea.


Example: (False Dichotomy)

Either restrictions must be placed on freedom of speech or certain subversive elements in society will use it to destroy this country. Since to allow the latter to occur is unconscionable, we must restrict freedom of speech.


The conclusion above is unsound because

  1. subversives do not in fact want to destroy the country.
  2. the author places too much importance on the freedom of speech.
  3. the author fails to consider an accommodation between the two alternatives
  4. the meaning of “freedom of speech” has not been defined
  5. subversives are a true threat to our way of life

The arguer offers two options: either restrict freedom of speech, or lose the country. He hopes the reader will assume that these are the only options available. This is unwarranted. He does not state how the so-called “subversive elements” would destroy the country, nor for that matter why they would want to destroy it. There may be a third option that the author did not mention; namely, that society may be able to tolerate the “subversives”; it may even be improved by the diversity of opinion they offer. The answer is (C).


Example: (False Dichotomy)

When workers do not find their assignments challenging, they become bored and so achieve less than their abilities would allow. On the other hand, when workers find their assignments too difficult, they give up and so again achieve less than what they are capable of achieving. It is, therefore, clear that no worker’s full potential will ever be realized.


Which one of the following is an error of reasoning contained in the argument?

  1. mistakenly equating what is actual and what is merely possible
  2. assuming without warrant that a situation allows only two possibilities
  3. relying on subjective rather than objective evidence
  4. confusing the coincidence of two events with a causal relation between the two
  5. depending on the ambiguous use of a key term

This argument commits the fallacy of false dichotomy. It assumes that workers have only two reactions to their work—either it’s not challenging or it’s too challenging. Clearly, there is a wide range of reactions between those two extremes. The answer is (B).



To score in the ninetieth percentile on the MCAT, one must study hard. If one studies four hours a day for one month, she will score in the ninetieth percentile. Hence, if a person scored in the top ten percent on the MCAT, then she must have studied at least four hours a day for one month.


Which one of the following most accurately describes the weakness in the above argument?

  1. The argument fails to take into account that not all test-prep books recommend studying four hours a day for one month.
  2. The argument does not consider that excessive studying can be counterproductive.
  3. The argument does not consider that some people may be able to score in the ninetieth percentile though they studied less than four hours a day for one month.
  4. The argument fails to distinguish between how much people should study and how much they can study.
  5. The author fails to realize that the ninetieth percentile and the top ten percent do not mean the same thing.

You may have noticed that this argument uses the converse of the fallacy “Confusing Necessary Conditions with Sufficient Conditions” mentioned earlier. In other words, it assumes that something which is sufficient is also necessary. In the given argument, this is fallacious because some people may still score in the ninetieth percentile, though they studied less than four hours a day for one month. Therefore the answer is (C).



Of course Steve supports government sponsorship of the arts. He’s an artist.


Which one of the following uses reasoning that is most similar to the above argument?

  1. Of course if a person lies to me, I will never trust that person again.
  2. Conservatives in the past have prevented ratification of any nuclear arms limitation treaties with the Soviet Union (or Russia), so they will prevent the ratification of the current treaty.
  3. Mr. Sullivan is the police commissioner, so it stands to reason that he would support the NRA’s position on gun control.
  4. Following her conscience, Congresswoman Martinez voted against the death penalty, in spite of the fact that she knew it would doom her chances for reelection.
  5. You’re in no position to criticize me for avoiding paying my fair share of taxes. You don’t even pay your employees a fair wage.

This argument is fallacious—and unfair—because it assumes that all artists support government sponsorship of the arts. Some artists, however, may have reasons for not supporting government sponsorship of the arts.


For example, they may believe that government involvement stifles artistic expression. Or they may reject government involvement on purely philosophical grounds. The argument suggests a person’s profession taints his opinion. Choice (C) does the same thing, so it is the answer.


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