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Suppressed Premises

Most arguments depend on one or more unstated premises. Sometimes this indicates a weakness in the argument, an oversight by the writer. More often, however, certain premises are left tacit because they are too numerous, or the writer assumes that his audience is aware of the assumptions, or he wants the audience to fill in the premise themselves and therefore be more likely to believe the conclusion.

Conclusion: I knew he did it.

Premise: Only a guilty person would accept immunity from prosecution.

The suppressed premise is that he did, in fact, accept immunity. The speaker assumes that his audience is aware of this fact or at least is willing to believe it, so to state it would be redundant and ponderous. If the unstated premise were false (that is, he did not accept immunity), the argument would not technically be a lie; but it would be very deceptive. The unscrupulous writer may use this ploy if he thinks that he can get away with it. That is, his argument has the intended effect and the false premise, though implicit, is hard to find or is ambiguous. Politicians are not at all above using this tactic.


Politician: A hawk should not be elected President because this country has seen too many wars.

The argument has two tacit premises—one obvious, the other subtle. Clearly, the politician has labeled his opponent a hawk, and he hopes the audience will accept that label. Furthermore, although he does not state it explicitly, the argument rests on the assumption that a hawk is likely to start a war. He hopes the audience will fill in that premise, thereby tainting his opponent as a warmonger.

A common question on the LSAT asks you to find the suppressed premise of an argument. Finding the suppressed premise, or assumption, of an argument can be difficult. However, on the LSAT you have an advantage—the suppressed premise is listed as one of the five answer-choices. To test whether an answer-choice is a suppressed premise, ask yourself whether it would make the argument more plausible. If so, then it is very likely a suppressed premise.

American attitudes tend to be rather insular, but there is much we can learn from other countries. In Japan, for example, workers set aside some time each day to exercise, and many corporations provide elaborate exercise facilities for their employees. Few American corporations have such exercise programs. Studies have shown that the Japanese worker is more productive than the American worker. Thus it must be concluded that the productivity of American workers will lag behind their Japanese counterparts, until mandatory exercise programs are introduced.

The conclusion of the argument is valid if which one of the following is assumed?
  1. Even if exercise programs do not increase productivity, they will improve the American worker’s health.
  2. The productivity of all workers can be increased by exercise.
  3. Exercise is an essential factor in the Japanese worker’s superior productivity.
  4. American workers can adapt to the longer Japanese work week.
  5. American corporations don’t have the funds to build elaborate exercise facilities.
The unstated essence of the argument is that exercise is an integral part of productivity and that Japanese workers are more productive than American workers because they exercise more. The answer is (C).

J. J. Thomson, the discoverer of the electron and a recipient of the Nobel Prize in physics, trained many physicists, among them seven Nobel Prize winners, 32 fellows of the Royal Society of London, and 83 professors of physics. This shows that the skills needed for creative research can be taught and learned.

Which one of the following is an assumption on which the argument depends?
  1. J. J. Thomson was an internationally known physicist, and scientists came from all over the world to work with him.
  2. All the scientists trained by J. J. Thomson were renowned for their creative scientific research.
  3. At least one of the eminent scientists trained by J. J. Thomson was not a creative researcher before coming to study with him.
  4. Creative research in physics requires research habits not necessary for creative research in other fields.
  5. Scientists who go on to be the most successful researchers often receive their scientific education in classes taught by renowned research scientists.
If the physicists trained by J. J. Thomson were creative researchers before studying under him, then clearly the argument would be specious. On the other hand, if none of the physicists were creative researchers before studying under Thomson, then the argument would be strong. However, the argument does not require this strong of a statement in order to be valid. All it needs is one person whose research skills profited from the tutelage of Thomson. The answer is (C).

Many students have problems with this type of question. They read through the answer-choices and find no significant statements. They may pause at (C) but reject it—thinking that the argument would be deceptive if only one person out of 122 profited from the tutelage of Thomson. However, the missing premise doesn’t have to make the argument good, just valid.

The petrochemical industry claims that chemical waste dumps pose no threat to people living near them. If this is true, then why do they locate the plants in sparsely populated regions. By not locating the chemical dumps in densely populated areas the petrochemical industry tacitly admits that these chemicals are potentially dangerous to the people living nearby.

Which of the following, if true, would most weaken the author's argument?
  1. Funding through the environmental Super Fund to clean up poorly run waste dumps is reserved for rural areas only.
  2. Until chemical dumps are proven 100% safe, it would be imprudent to locate them were they could potentially do the most harm.
  3. Locating the dumps in sparsely populated areas is less expensive and involves less government red tape.
  4. The potential for chemicals to leach into the water table has in the past been underestimated.
  5. People in cities are more likely to sue the industry if their health is harmed by the dumps.
The suppressed false premise of the argument is that all things being equal there is no reason to prefer locating the sites in sparsely populated areas. To weaken the argument, we need to show it is not true that all things are equal. In other words, there are advantages other than safety in locating the sites in sparsely populated areas. Choice (C) gives two possible advantages—cost and ease. Hence (C) is the answer.

The following example has a hidden true premise.
Example (Suppressed true premise.)
The news media is often accused of being willing to do anything for ratings. However, recent action by a television network indicates that the news media is sometimes guided by moral principle. This network had discovered through polling voters on the east coast that the Republican candidate for President had garnered enough votes to ensure victory before the polls closed on the west coast. However, the network withheld this information until the polls on the west coast closed so that the information would not affect the outcome of key congressional races.

Which one of the following most strengthens the argument?
  1. The network had endorsed the Republican candidate for President.
  2. The network expected its ratings to increase if it predicted the winner of the presidential race, and to decrease if did not predict the winner.
  3. A rival network did predict a winner of the presidential race before the polls on the west coast closed.
  4. The network believed that it would receive higher ratings by not predicting the winner of the presidential race.
  5. The network feared that predicting the winner of the presidential race could so anger Congress that it might enact legislation preventing all future polling outside of voting centers.
The suppressed premise in this argument is that the network hurt itself by not predicting the winner of the presidential race, or at least did not help itself. To strengthen the argument, we need to show that this assumption is true. Choice (B) implies that this is the case by stating that the network expected to lose ratings if it did not predict a winner. Hence, the answer is (B).

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