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.