The suggestion has already been made that no matter how perfect an analogy may be, it can never amount to absolute proof. At its best analogy creates only a high degree of probability. In order to strengthen the conclusion a diligent search should be made for other lines of reasoning which will fortify it. One of the most important uses to which analogy may be put is to suggest possible conclusions which may be substantiated by other processes of reasoning, as induction, deduction, or causal relation. If two or more lines of reasoning can be made to support the same conclusion the probability of its truth is greatly strengthened; hence its argumentative value is increased. Where all available processes of reasoning may be made to establish one conclusion the probability of its truth is so strengthened that it amounts to moral certainty, but no cumulation of probabilities can ever amount to absolute certainty.
The fact that analogy must be substantiated by other processes of reasoning should not lead the student to underestimate its importance. The examples and explanations which have been given should lead him to appreciate fully the fact that analogy has two well defined uses aside from its value as proof of the truth or falsity of a conclusion. In the first place it is a most important agency in suggesting conclusions which may be verified or discredited by other processes of reasoning. In the second place it affords a most valuable method of stating a case so plainly that even the most ignorant may understand. A striking analogy makes a most vivid impression on the mind and is retained long after more formal processes of reasoning are forgotten.
SUMMARY OF THE REQUIREMENTS FOR AN ARGUMENT FROM ANALOGY
I. The two factors in the analogy must be alike in all particulars which affect the conclusion.
II. The alleged facts upon which the analogy is based must be true.
III. The conclusion established by analogy should whenever possible be verified by positive evidence.
EXERCISES IN ANALOGY
I. Apply the requirements for validity to each of the arguments from analogy quoted in this last chapter.
II. Suggest arguments from analogy in support of each of the following conclusions:
1. College students should be allowed to select their own courses of study.