"Mr. Pinchot compared our present consumption of wood to the case of a man in an open boat at sea, cut adrift from some shipwreck and with but a few days' supply of water on board. He drinks all the water the first day, simply because he is thirsty, though he knows that the water will not last long. The American people know that their wood supply will last but a few decades. Yet they shut their eyes to the facts."
Water and wood are not alike in themselves; they cannot be directly compared, but they are alike in the relations they bear to other circumstances.
When President Lincoln refused to change generals at a certain time during the Civil War, saying that it was not wise to "swap horses while crossing a stream," he reasoned from analogy. Since the horse in taking its master across the stream and the general in conducting a campaign are totally unlike in themselves but have similar relations, the argument is from analogy and not from generalization.
It is easy to see that such reasoning never constitutes indubitable proof. If argument from generalization, where the objects compared differ from each other in only a few respects, is weak, plainly, argument from analogy is much weaker, since the objects are alike merely in the relations they bear.
Though argument from analogy does not constitute proof, yet it is often valuable as a means of illustration. Truths frequently need illumination more than verification, and in such cases this sort of comparison may be very useful. Many proverbs are condensed arguments from analogy, their strength depending upon the similarity between the known case and the case in hand. It is not hard to find the analogy in these expressions: "Lightning never strikes twice in the same place"; "Don't count your chickens before they are hatched"; "A fool and his money are soon parted."
The student who has carefully read this chapter up to this point should have a fairly clear idea of the nature of proof; he should know that proof consists of evidence and reasoning; he should know the tests for each of these; and he should be able to distinguish between strong and weak arguments. The next step for him to take will be to apply these instructions in generating proof for any statement that he wishes to establish.
A common fault in argumentation is the failure to support important points with sufficient proof. One or two points well established will go farther toward inducing belief in a proposition than a dozen points that are but weakly substantiated. A statement should be proved not only by inductive reasoning, but, if possible, by deductive. If one uses argument from antecedent probability in establishing a statement, he should not rest content with this one method of proof, but he should try also to use argument from sign, and argument from example, and, whenever he can, he should quote authority.
Notice that in the following outline three kinds of proof are used. The amount of proof here given is by no means sufficient to establish the truth of the proposition being upheld; the outline, however, does illustrate the proper method of building up the proof of a proposition.
The present condition of the United States Senate is deplorable.