The theory and practice of argumentation and debate

by Victor Alvin Ketcham

3. Therefore John Pitt ought to be a candidate for the class football team.

Upon examining the major premise we find that it is not true as a matter of fact, because it is obvious that one who is not physically capable of taking part in such a game ought not to do so even though he is a sophomore. The deduction is therefore fallacious. But suppose the major premise to be sound, the next step in the search for fallacies would be to examine the minor premise and find out whether it is true as a matter of fact. An examination of this premise may disclose the fact that John Pitt is a junior. The deduction is therefore fallacious, because the minor premise is not true as a matter of fact. A fallacy in a deductive argument may be exposed by showing that either the major premise or the minor premise is not true as a matter of fact.

2. Logical fallacies.

We now come to the class of fallacies which inhere in deductive reasoning independent of the truth or falsity of the alleged facts contained in the premises. These are called logical fallacies. They consist of many forms of error in reasoning, but we shall concern ourselves only with those most likely to be encountered. These are four in number, (1) The undistributed middle, (2) The illicit process, (3) Irrelevancy of premises, or ignoring the question, and (4) Begging the question.

(1). The undistributed middle.

One of the most common errors of deductive argument is called the fallacy of the undistributed middle. It consists of a defect in the major premise. This defect is the failure of the major term to include the middle term. The following syllogism is a typical illustration of this error:

1. Some college men are successful in business.

2. Henry Winslow is a college man.

3. Therefore Henry Winslow is successful in business.

The student will observe that the major term, “men who are successful in business,” does not include the middle term, “college men,” but only includes a part of that class of men. This is true because the middle term reads “Some college men.” Therefore it is evident that there are some college men who are not successful in business as well as some who are. To represent this defect graphically the device of circles employed in discussing the construction of valid deductions may again be used. The result is as follows: