The human conscience is so constituted that many people deviate from the truth for no apparent reason whatever. Some are given to exaggeration; some habitually pretend to know that of which they are entirely ignorant; others are so inaccurate that everything they say is open to grave suspicion. If a witness is known to have been repeatedly dishonest or inaccurate in the past, little reliance should be placed in his testimony. "Yellow journalism," which is largely the reflection of common rumor, affords constant examples of witnesses that give questionable evidence.
Ability and willingness to give exact evidence, an unprejudiced attitude, and a good reputation for honesty and accuracy are the qualities that should characterize the sources of evidence. If a writer or speaker is securing testimony from friends or acquaintances, the application of these tests is not difficult. If, however, the sources are books and periodicals, his work is harder; but to be successful, he must not shirk it. When one procures evidence from books, he should investigate the character and standing of the author. When one obtains it from signed articles in papers and magazines, he must consider both the author and the character of the publication. In the case of newspaper "stories" and editorials, one should find out on what general policy and principles the paper is conducted. A cautious arguer will always avoid, as far as he can, the use of evidence that comes from a doubtful source. If one finds that an opponent has used the testimony of questionable witnesses, he can, by exposing the fact, easily refute the argument.
NECESSITY OF STATING SOURCES. It sometimes happens that an arguer fails to state the source of his evidence. This omission is usually fatal to success. No one is likely to put much confidence in statements that are introduced by such flimsy preambles as, "A certain statesman has declared"; "I have read somewhere"; "An acquaintance told me." Not only must evidence come from sources that seem good to the writer, but those sources must be satisfactory to the audience. In the last analysis the audience is the judge of what is credible and what is not. Moreover, if the evidence is of great importance, or is liable to be disputed, the arguer should show in a few words why the witness is especially reliable.
B. INTERNAL TESTS OF EVIDENCE
(1) Is the evidence consistent with (a) other evidence in the same argument; (b) known facts; (c) human experience?
The requirement that every separate bit of evidence in an argument shall be consistent with every other bit of evidence in the same argument is too well understood to need explanation. One familiar with courts of law knows that a witness who contradicts himself is not believed. Furthermore, if the testimony of several witnesses for the same side is inconsistent, the case for that side is materially weakened. So it is in general debate: the arguer who wishes to succeed must not use evidence that is self-contradictory. His proof must "hang together"; his facts must all go to establish the same conclusion.
A flagrant violation of this principle once occurred in a class-room debate. The speaker for the negative on the proposition, "Resolved, That freshmen should be ineligible for college teams," said that such a rule would deprive the freshmen of much- needed physical exercise. Later on, he said that just as many freshmen would receive injuries under this rule as without it, since they would take part in equally dangerous contests as members of freshmen teams. This contradiction ruined his argument.
In the next place, evidence to be of any value whatever, must be consistent with what is known about the case. If an arguer is so careless as to make statements contradictory either to well- established facts or to facts easily proved, he cannot hope to attain the slightest measure of success. Only one guilty of gross neglect or absolute falsehood is likely to fall into such an error. At one time the story was circulated that, during his early life, Lincoln had been insane. In the following passage Ida M. Tarbell shows that the testimony on which this belief was founded is inconsistent with the known facts of the case, and is, therefore, palpably untrue:—
"Mr. Thornton went on to say that he knew beyond a doubt that the sensational account of Lincoln's insanity was untrue, and he quoted from the House journal to show how it was impossible that, as Lamon says, using Herndon's notes, 'Lincoln went crazy as a loon, and did not attend the legislature in 1841-1842, for this reason'; or, as Herndon says, that he had to be watched constantly. According to the record taken from the journals of the House by Mr. Thornton, which have been verified in Springfield, Mr. Lincoln was in his seat in the House on that 'fatal first of January' when he is asserted to have been groping in the shadow of madness, and he was also there on the following day."
Lincoln himself was an expert at detecting inconsistency wherever it existed. He won many of his lawsuits by the straightforward method of showing that the one or two vital statements on which the whole case of the opposition rested were false, inasmuch as they were inconsistent with well-established and incontrovertible facts. An instance of this sort is here described:—