EXERCISES
A. Supply the missing premise for the following:—
1. John will succeed because he has a college education.
2. Henry is happy because he has plenty of money.
3. Candy is nutritious because it is made of sugar.
4. These biscuits will make me ill because they are heavy.
5. This dog must be angry because he is growling.
6. This fish can swim.
7. The plural of the German noun der Garten is die Gärten.
8. It will hurt to have this tooth filled.
B. Supply the reasons and complete the syllogism for each of the following:—
1. This book should not be read. 2. This hammer is useful. 3. That dog will bite. 4. This greyhound can run rapidly. 5. The leaves have fallen from the trees. 6. That boy ought to be punished. 7. It is too early to go nutting. 8. This boy should not study. 9. You ought not to vote for this man for mayor.
+Theme XCVIII.+—Write a paragraph proving the truth of one of the following propositions:—
1. Labor-saving machinery is of permanent advantage to mankind.
2. New Orleans will some day be a greater shipping port than New York.
3. Poetry has a greater influence on the morals of a nation than prose writing.
4. Boycotting injures innocent persons and should never be employed.
5. Ireland should have Home Rule.
6. The President of the United States should be elected by the direct vote of the people.
(Consider your argument with reference to the suppressed premises.)
+180. Errors of Deduction.+—The deductive method of reasoning, if properly used, is effective, but much care needs to be taken to avoid false conclusions. A complete exposition of the variations of the syllogism is not necessary here, but it will be of value to consider briefly three chief errors.
If the terms are not used with the same meaning throughout, the conclusion is valueless. A person might agree with you that domestic arts should be taught to girls in school, but if you continued by saying that scrubbing the floor is a form of domestic art, therefore the girls should be taught to scrub the floor, he would reject your conclusion because the meaning of the term domestic art as he understood it in the first statement, is not that used in the second.
It will be noticed that each syllogism includes three terms. For example, the syllogism,—
All hawks eat flesh;
This bird is a hawk;
Therefore this bird eats flesh,—
contains the three terms, hawk, eats flesh, this bird; of these but two appear in the conclusion. The one which does not (in this case hawk) is called the middle term. If the major premise does not make a statement about every member of the class denoted by the middle term, the conclusion may not be valid even though the premises are true. For example:—
All hawks are birds;
This chicken is a bird;
Therefore this chicken is a hawk.
In this case the middle term is birds, and the major premise, All hawks are birds, does not make a statement which applies to all birds. The conclusion is therefore untrue. Such an argument is a fallacy.
The validity of the conclusion is impaired if either premise is false. In the enthymeme, "Henry is a coward; he dare not run away from school," the suppressed premise, "All persons who will not run away from school, are cowards," is not true, and so invalidates the conclusion. It is well to test the validity of your own argument and that of your opponent by seeking for the suppressed premise and stating it, for this may reveal a fatal weakness in the thought.