Because of the divergencies in the results produced by reasoning, a student should study with considerable care the various processes of arriving at a conclusion, so that he may be able to tell what methods are strong, what are weak, and what are fallacious.
According to a common classification, there are two methods of reasoning: the inductive process, and the deductive process.
1. INDUCTIVE REASONING. When one carefully investigates his reasons for believing as he does, he often finds that he accepts a certain statement as true because he is familiar with many specific instances that tend to establish its truth. The belief that prussic acid is poisonous is based upon the large number of instances in which its deadly effect has been apparent. The fact that railroad men are exposed to injury is unquestioned because every one is familiar with the many accidents that occur each year. The statement that water freezes at thirty-two degrees Fahrenheit has been proved true by innumerable tests. This process of reasoning by which, from many specific instances, the truth of a general statement is established, is called induction.
An example of inductive reasoning is found in the following passage:—
Does the closing of the saloons affect appreciably the amount of drunkenness in the community? A comparison of the same town or city in successive years—one year under one system, and the next year under the other—furnishes a basis for accurate judgment. Evidence of this sort is all one way, and it seems to be conclusive.
The tables prepared by the Massachusetts Bureau of Statistics of Labor, in 1905, under special instructions from the legislature, show that in Haverhill the average number of arrests per month under license was 81.63, under no-license, 26.50; in Lynn, under license, 315, under no-license, 117.63; in Medford, under license, 20.12, under no-license, 13.25; in Pittsfield, under license, 93.25, under no-license, 36.75; and in Salem, under license, 140.50, under no-license, 29.63. Such comparisons might be multiplied, but it is unnecessary. There is no escaping the conclusion that the closing of the saloons, under the Local Option system, does sensibly diminish the volume of drunkenness. [Footnote: Atlantic Monthly, Vol. XC, p. 437.]
In using inductive reasoning, one must always be on his guard against drawing conclusions too hastily. It is never correct to conclude from a consideration of only a few instances that a general truth has been discovered. Further examination may show that the opinion first formed will not hold. Some people call all men dishonest because several acquaintances have not kept faith with them. Others are ready to believe that because they have made money in the stock market all can do likewise. Most superstitions arise through generalization from too few instances: those who have several times met misfortune on the thirteenth day of the month are apt to say that the thirteenth is always an unlucky day. Such reasoning as this shows the weakness of inductive argument: a conclusion is worthless if it is drawn from too few examples.
Professor Fred Lewis Pattee, in writing on Errors in Reasoning, says:—
Children and even adults often generalize from a single experience. A little boy cautioned me at one time to keep away from a certain horse, for "white horses always kick." An old Pennsylvania farmer laid down the law that shingles laid during the increase of the moon always curl up. He had tried it once and found out. A friend will advise you to take Blank's Bitters: "I took a bottle one spring and felt much better; they always cure." Physicians base their knowledge of medicines upon the observations of thousands of trained observers through many years, and not upon a single experience. Most people are prone to judge their neighbors from too slight acquaintance. If a man is late at an appointment twice in succession, someone is sure to say: "Oh, he's always late." This is poor thinking because it is bad judgment. Judgments should be made with care and from fullness of experience. [Footnote: The Adult Bible Class and Teacher Training Monthly, May, 1908, page 295.]
The following quotation illustrates how often hasty generalizations create prejudice and sway public judgment:—