The theory and practice of argumentation and debate

by Victor Alvin Ketcham

4. A search should be made for exceptions to the rule stated by the induction.

One of the most effective ways to overthrow a generalization is to present exceptions. Even the existence of one exception will greatly weaken the effect of a conclusion, while several exceptions, clearly established, will entirely destroy it. To prove the existence of more exceptions to the rule than there are instances supporting it is to prove it entirely fallacious. The search for exceptions should be made by the same means employed in finding instances to support the induction. The fallacy of an induction may be shown by proving the existence of exceptions to the rule which it states.

5. The induction should be examined with a view to determining its reasonableness.

An induction which appears on its face to be contrary to usual experience is not an effective instrument of persuasion. By showing that it is contrary to natural law or that no process of reasoning other than induction can be made to uphold it, the student may weaken its force. If clear proof of its validity can be established in this way it is not necessary that other methods of showing a fallacy be introduced. The fallacy of an induction may be established by clear proof of its unreasonableness.

II. Fallacies of deduction.

A thorough study of the chapter on Deductive Argument has revealed the fact that such an argument in order to be valid must be constructed according to certain definite principles. The knowledge of these principles thus acquired should enable the student to detect fallacies in this form of argument. Nevertheless, some of the fallacies to which deduction is liable are so important and so easily concealed that a separate treatment of them is necessary. Fallacies of deduction may be divided into two classes, 1. Material fallacies, and 2. Logical fallacies.

1. Material Fallacies.

We have already learned that the deductive argument is seldom found in the form of a syllogism but is mostly encountered in the form of an enthymeme, which must be reduced to the syllogistic form. The method of reduction has been explained in the chapter on Deductive Argument and exercises in the use of that method have been given. It is therefore assumed that the student is so familiar with this process that he can readily reduce any argument to the syllogistic form. In the search for fallacies we may begin at this point. After the argument has been reduced to syllogistic form our first task is to examine the major and minor premises for the purpose of discovering any material error, or error of fact. In constructing our own argument we have been cautioned to see to it that both of these statements in the syllogism are true. Now we are examining our own arguments or our opponents’ arguments for the very purpose of finding out whether they contain any error. A sophomore urges John Pitt to come out for the class football team by saying that all sophomores ought to be candidates for places on the team. Reduced to the syllogistic form the argument would stand as follows:

1. All sophomores ought to be candidates for the class football team.

2. John Pitt is a sophomore.