Assumed Premises

An argument is a relationship between a set of premises and a conclusion. But people rarely express in words all the premises they are using. Most arguments contain some premises that are assumed but not stated, implicit rather than explicit.

Example:

Suppose we are planning a hiking trip, and I tell you that Sally can't come because she has a broken leg. My argument clearly assumes that people with broken legs can't go hiking, but I didn't state that premise, because it was too obvious.

When we analyze an argument, it is important to identify the assumed premises. They can then be labeled--using letters instead of numbers to distinguish them from explicit premises-- and included in the diagram.

There are two basic rules we should follow in interpreting or analyzing an argument by providing assumed premises:

1. The premise we supply should close the logical gap between the stated premise and the conclusion, and
2. The premise we supply should not commit us to more than is necessary.
Example:

Suppose you are taking French, and you learn that some of your classmates are failing; you infer they they do not enjoy the subject. The task, now, is to identify the assumed premise in your argument.

Consider the following candidates:

A. French is a Romance language.
B. Paris is beautiful in the springtime.
C. People never enjoy something that they find difficult.
D. Students do not enjoy subjects in which they are failing.

Using rule 1, I can eliminate candidates A and B. Neither of them helps close the gap in the argument as stated; neither premise is relevant to the conclusion.

Rule 2 helps me choose between the remaining candidates. Notice that C is a much more sweeping generalization than D; C applies to all people and all activities, whereas D applies only to students and the subjects they are studying. Because premise D closes the gap in the argument, without committing me to as much as C does, it is the one I should use.


Comprehension Questions

1 Identify the assumed premise in the following argument.

President Clinton will not be successful in Bosnia because he is a rookie at war.

a) President Clinton will not be successful in Bosnia.
b) No one who took a deferment from military service can be successfully conduct a war.
c) No one will be successful in Bosnia.
d) No one who is a rookie at war will be successful in Bosnia.
2 Identify the assumed premise in the following argument.

A heroic movie might induce more violence than a slasher one in which the villan is punished for his violence because the hero in heroic movies is rewarded for his violence.

a) The hero of a heroic movie is rewarded for his violence.
b) A heroic movie might induce more violence than slasher ones in which the villan is punished for his violence.
c) Any movie that rewards violence encourages more violence than one that punishes it.
d) Punishing the villans of a violent movie is a way to prevent violence among moviegoers.
3 Identify the assumed premise in the following argument.

Fetal studies have shown that individuals are initially biased toward right-handedness. However, a recent study showed that there were more left-handed individuals among fetuses subject to stress, such as prolonged labor or undernourishment, then would be expected.

a) Stress causes individuals to switch from being right-handed to being left-handed.
b) Researchers hadn't done enough research on the factors influencing handedness until the most recent studies were done.
c) Stress causes brain damage in fetuses.
d) Genes, rather than environmental factors, determine handedness.


Assessing Logical Strength

© Copyright 1998, W.W. Norton & Co.