Circumstantial Evidence. In most cases in court such direct evidence is the exception rather than the rule, for a man attempting crime would shun circumstances in which his crime would be witnessed. Indirect evidence—circumstantial evidence—is much more usual. It lacks the certainty of direct evidence, yet from the known facts presented it is often possible to secure almost the same certainty as from direct evidence. In serious crimes, such as murder, juries are extremely cautious about convicting upon circumstantial evidence. There are many chances of error in making chains of evidence. In indirect evidence a group of facts is presented from which a conclusion is attempted. Suppose a boy had trouble with a farmer and had been heard to threaten to get even. One day the man struck him with a whip as he passed on the road. That night the farmer's barn was set on fire. Neighbors declared they saw some one running from the scene. Next day the boy told his companions he was glad of the loss. Circumstantial evidence points to the boy as the culprit. Yet what might the facts be?
In presenting arguments get as much direct evidence as possible to prove your statements. When direct evidence cannot be secured, link your indirect evidence so closely that it presents not a single weak link. Let the conclusion you draw from it be the only possible one. Make certain no one else can interpret it in any other way.
When you present evidence be sure it completely covers your contention. Be sure it is clear. Be sure it fits in with all the other facts and details presented. Do not let it conflict with usual human experience. Consider the sources of your evidence. If you do not, you can be certain your audience will. Are your sources reliable? Is the information authoritative? Is it first-hand material, or merely hearsay? Is it unprejudiced? Many of the other facts for evidence have already been suggested in the chapter on getting material.
Two General Methods of Reasoning. Frequently the evidence to be used in argumentation must be interpreted before it can be of any value, especially when dealing with propositions of expediency or policy. There are two general methods of reasoning. One is the inductive method, the other the deductive.
Inductive Reasoning. When we discover that a certain operation repeated many times always produces the same result we feel justified in concluding that we can announce it as a universal law. After thousands of falling bodies have been measured and always give the same figures, scientists feel that they may state the law that all falling bodies acquire an acceleration of 32.2 feet per second. This illustrates the inductive method of reasoning. In this system we reason from the specific instance to the general law, from the particular experiment to the universal theory, from the concrete instance to the wide principle.
All modern science is based upon this method—the experimental one. All general theories of any kind today must—to be accepted—be supported by long and careful consideration of all possible and probable circumstances. The theory of evolution as applied to the living things upon the earth is the result of countless observations and experiments.
Hasty Generalization. The speaker cannot himself examine all the specific instances, he cannot consider all the illustrations which might support his position, but he must be careful of a too hasty generalization. Having talked with a dozen returned soldiers he may not declare that all American army men are glad to be out of France, for had he investigated a little further he might have found an equal number who regret the return to this land. He must base his general statement on so many instances that his conclusion will convince not only him, but people disposed to oppose his view. He must be better prepared to show the truth of his declaration than merely to dismiss an example which does not fit into his scheme by glibly asserting that "exceptions prove the rule." He must show that what seems to contradict him is in nature an exception and therefore has nothing at all to do with his rule. Beginning speakers are quite prone to this fault of too hasty generalization.
EXERCISES
1. Write down five general theories or statements which have been established by inductive reasoning.