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 cla.s.s 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.
EXERCISES
Which of the following are incorrect?
1. The government should pay for the education of its people; Travel is a form of education; Therefore the government should pay the traveling expenses of the people.
2. All horses are useful; This animal is useful; Therefore this animal is a horse.
3. I ought not to study algebra because it is a very difficult subject.
4. Pupils ought not to write notes because note writing interferes with the rights of others.
5. All fish can swim; Charles can swim; Therefore Charles is a fish.
6. Henry is a fool because he wears a white necktie.
7. All dogs bark; This animal barks; Therefore this animal is a dog.
+Theme XCIX.+--_Write a paragraph proving the truth of one of the following propositions:_--
1. The government should establish a parcels post.
2. The laws of mind determine the forms of composition.
3. Training for citizenship should be given greater attention in the public schools.
4. The members of the school board should be appointed by the mayor of the city.
5. In the estimation of future ages ---- will be considered the greatest President since Lincoln.
(State your premises. Have you shown that they are true?)
+181. Evidence.+--We may reach belief in the truth of a specific statement by means of deductive reasoning. Commonly, however, when dealing with an actual state or occurrence, we present other facts or circ.u.mstances that show its existence. The facts presented may be those of experience, the testimony of witnesses, the opinion of those considered as experts in the subject, or a combination of circ.u.mstances known to have existed. To be of any value as arguments, they must be true, and they must be related to the fact that we are trying to prove. These true and pertinent facts we term _evidence_.
Evidence may be direct or indirect. If a man sees a boy steal a bag of apples from the orchard across the way, his evidence is direct. If instead, he only sees him with an empty bag and later with a full one, the evidence will be indirect. If you testify that early in the evening you saw a tramp enter a barn which later in the evening caught fire, your testimony as regards the cause of the fire would be indirect evidence against the tramp. If you can testify that you saw sparks fall from his lighted pipe and ignite a pile of hay in the barn, the evidence which you give will be direct.
Direct evidence has more weight than indirect, but often the latter is nearly equal to the former and is sufficient to convince us. Even the direct testimony of eye-witnesses must be carefully considered. Several persons may see the same thing and yet make very different reports, even though they may all desire to tell the truth. The weight that we shall give to a person"s testimony will depend upon his ability to observe and to report accurately what he has experienced, and upon his desire to tell the truth.
Notice in the following selection what facts, specific instances, and circ.u.mstances are advanced in support of the proposition. a.s.suming that they are true, are they pertinent to the proposition?
Certain species of these army ants which inhabit tropical America, Mr.
Belt considered to be the most intelligent of all the insects of that part of the world. On one occasion he noticed a wide column of them trying to pa.s.s along a nearly perpendicular slope of crumbling earth, on which they found great difficulty in obtaining a foothold. A number succeeded in retaining their positions, and further strengthened them by laying hold of their neighbors. They then remained in this position, and allowed the column to march securely and easily over their bodies. On another occasion a column was crossing a stream of water by a very narrow branch of a tree, which only permitted them to go in single file. The ants widened the bridge by a number clinging to the sides and to each other, and this allowed the column to pa.s.s over three or four deep. These ants, having no permanent nests, carry their larvae and pupae with them when marching. The prey they capture is cut up and carried to the rear of the army to be distributed as food.
--Robert Brown: _Science for All_.
+Theme C.+--_Present all the evidence you can either to prove or disprove one of the following propositions:_--
Select some question of local interest as:-- 1. The last fire in our town was of incendiary origin.
2. The football team from ---- indulged in "slugging" at the last game.
3. Our heating system is inadequate.
4. It rained last night.
If you prefer, choose one of the following subjects:-- 1. The Stuart kings were arbitrary rulers.
2. The climate of our country is changing.
3. Gutenberg did not invent the printing press.
4. The American Indians have been unjustly treated by the whites.
5. Nations have their periods of rise and decay.
(Are the facts you use true? Are they pertinent? Do you know of facts that would tend to show that your proposition is not true?)
+182. Number and Value of Reasons.+--Although a statement may be true and pertinent it is seldom sufficient for proof. We need, as a rule, several such statements. If you are trying to convince a friend that one kind of automobile is superior to another, and can give only one reason for its superiority, you no doubt will fail in your attempt. If, however, you can give several reasons, you may succeed in convincing him. Suppose you go to your princ.i.p.al and ask permission to take an extra study. You may give as a reason the fact that your parents wish you to take it. He may not think that is a sufficient reason for your doing so, but when he finds that with your present studies you do not need to study evenings, that one of them is a review, and that you have been standing well in all your studies, he may be led to think that it will be wise for you to take the desired extra study.