There are two divisions of argument from example. When the resemblance between the things compared is close, the process is called argument by generalization; when the resemblance is so slight that there can be no direct comparison, but only a comparison of functions, the process is called argument from analogy.
ARGUMENT BY GENERALIZATION. If one finds that a certain mastiff becomes with training an excellent watch dog, he may reasonably take it for granted that training will produce the same result in another dog of the same breed. If a college student with certain pronounced physical and mental characteristics is known to be an exceptionally good football player, the athletic trainer is sure to reason by generalization that another student with these same characteristics would be a valuable addition to the team. Burke in his Speech on Conciliation uses this kind of reasoning when he says that just as Turkey and Spain have found it necessary to govern their distant possessions with a loose rein, so, too, England will be obliged to govern the American Colonies leniently.
Benjamin Harrison used this method of argument in the following quotation:—
That we give back to Porto Rico all the revenue derived from the customs we levy, does not seem to me to soften our dealings with her people. Our fathers were not mollified by the suggestion that the tea and stamp taxes would be expended wholly for the benefit of the colonies. It is to say: We do not need this money; it is only levied to show that your country is no part of the United States, and that you are not citizens of the United States, save at our pleasure. [Footnote: North American Review, January, 1901, p. 17.]
Argument by generalization very rarely constitutes absolute proof. In dealing with things, it may do so in rare cases; in dealing with human actions, almost never. The reason why it can establish only a strong probability lies in a weakness in the process of reasoning.
Notice that while this kind of argument apparently reasons directly from the example cited to the case in hand, there is in reality an intermediate step. This step is a general truth of which both the known fact and the fact to be proved must be instances. When it is argued that since one mastiff makes a good watch dog another mastiff will also make a good watch dog, the reasoning passes through the general statement, "All mastiffs make good watch dogs."
Graphically the process might be represented thus:—
General Law
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/ \
/ \
/ \
Known Fact Fact to be Proved
This method is very much like the method of reasoning from effect to effect, except that here the intermediate step does not cause, but merely accounts for the facts. In the illustration taken from Burke, the known fact is that neither Turkey nor Spain can govern their distant provinces despotically. The general law is that no country can govern a distant dependency harshly. The fact proved is that England cannot play the despot with the American Colonies.
The weakness of this sort of reasoning is now easily seen. In the first place, there are few general laws governing human action that always hold true. In the second place, unless there is a very strong resemblance between the cases compared, unless they are alike in all essential particulars, they will not both be examples of the working of one general law.