Practical Argumentation · Chapter 11B. Internal Tests of Evidence
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(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:—
The most damaging evidence was that of one Allen, who swore that he had seen Armstrong strike Metzker about ten or eleven o'clock in the evening. When asked how he could see, he answered that the moon shone brightly. Under Lincoln's questioning he repeated the statement until it was impossible that the jury should forget it. With Allen's testimony unimpeached, conviction seemed certain.
Lincoln's address to the jury was full of pathos. It was not as a hired attorney that he was there, he said, but to discharge a debt of friendship…. But Lincoln was not relying on sympathy alone to win his case. In closing he reviewed the evidence, showing that all depended on Allen's testimony, and this he said he could prove to be false. Allen never saw Armstrong strike Metzker by the light of the moon, for at the hour when he said he saw the fight, between ten and eleven o'clock, the moon was not in the heavens. Then procuring an almanac, he passed it to the judge and jury. The moon, which was on that night only in its first quarter, had set before midnight. [Footnote: The Life of Abraham Lincoln, Vol. I, p. 272. Ida M. Tarbell. The Doubleday & McClure Co.]
An arguer should also be extremely careful to use evidence that on its face appears reasonable. Only an extremely credulous audience will accept ideas that run counter to human belief and experience. To attribute the occurrence of an event to supernatural causes would bring a smile of derision to any but a most ignorant and superstitious person. To attribute to men qualities and characteristics that human experience has shown they do not possess will bring equal discredit. No one is likely to accept evidence that contradicts his habits of thinking, that is contrary to what his life and experience have taught him is true. For this reason savage people are slow to believe the teachings of the Christian religion. For this reason it is difficult to make an audience believe that any one will deliberately and consistently work against his own interests, or follow any other unusual line of action. Evidence contrary to human experience may be true, but unless the exigencies of the argument demand its use, the arguer will do well to omit it entirely. If he is obliged to use it, he should make it appear as reasonable as he can, and also substantiate it with careful proof.
Huxley appreciated the fact that evidence, to be believed, must be in accordance with man's experience when he wrote the following:—
If any one were to try to persuade you that an oyster shell (which is also chiefly composed of carbonate of lime) had crystallized out of sea-water, I suppose you would laugh at the absurdity. Your laughter would be justified by the fact that all experience tends to show that oyster-shells are formed by the agency of oysters, and in no other way.
The ease with which an argument that does not satisfy this requirement may be overthrown is clearly shown in the following extract from a student's forensic:—
To say that the Cuban reconcentrados sunk the Maine in an effort to embroil the United States in a conflict with Spain is the veriest foolishness. There is not one scrap of documentary evidence to show that such was the case. Moreover, such an act would be unparalleled in the annals of history. It is unreasonable, contrary to all experience, that those oppressed people should have brought disaster, involving the destruction of property and the loss of many lives, upon the very nation that they were looking to for assistance.
(2) Is the evidence first-hand or hearsay evidence?
It is universally recognized that hearsay evidence is unreliable. A narrative is sure to become so garbled by passing from mouth to mouth that unless a witness can testify to a fact from his own personal knowledge the evidence he gives is worthy of little credence. There is sufficient chance for error when the person who witnessed the event relates the account himself; if the story is told by a second, and perhaps by a third person, it is likely to reflect but little of what really happened. Every one is familiar with the exaggerations of common rumor; it distorts facts so that they are unrecognizable. The works of Herodotus are untrustworthy because he frequently believed hearsay evidence. Since second-hand evidence both fails to establish anything worth while, if allowed to stand, and is easily overthrown even by a very little first-hand evidence, an arguer will do well to follow the custom of the law courts, and, as a rule, exclude it altogether.
(3) Can the evidence be considered as especially valuable?
(a) Hurtful admissions constitute an especially valuable kind of evidence. Since men are not wont to give evidence detrimental to their personal interest unless impelled to do so by conscientious scruples, any testimony damaging to the one who gives it is in all probability not only truthful, but also the result of careful investigation. When a practising physician admits that half the ailments of mankind are imaginary or so trivial as to need no medical attention, he is making a statement that is likely to injure his business; for this reason he is probably stating the result of his experience truthfully. If a railroad president says that in his opinion government supervision of railroads will benefit the public in the matter of rates and service, it may be taken for granted that he has given his honest belief, and that his natural reluctance to surrender any authority of his own has kept him from speaking carelessly. If a member of the United States Senate admits that that body is corrupt, and selfish, and untrustworthy, he is lowering his own rank; therefore it is reasonable to believe that he is speaking the truth according to his honest belief.
The following is an example of this kind of evidence:—
It was stated during the Manchurian campaign that the Jewish soldiers, of whom Kuropatkin had about 35,000, not only failed to hold their ground under fire, but by their timidity threw their comrades into panic. But good evidence can be cited from the correspondents of the Novoye Vremya, an Anti-Semitic organ, to the effect that among the Jews were found many "intrepid and intelligent soldiers," and that a number of them were awarded the St. George's cross for gallantry. [Footnote: The Nation, June 11, 1908.]
It is hardly necessary to add that one who places especial reliance on this kind of evidence must be sure that the admission is really and not merely apparently contrary to the interest of the one who gives it.
(b) Another particularly valuable kind of evidence is negative evidence, or the evidence of silence. Whenever a witness fails to mention an event which, if it had occurred, would have been of such interest to him that he might reasonably have been expected to have mentioned it, his silence upon the matter becomes negative evidence that the event did not occur. For many years no one suggested that Bacon wrote the Shakespearean plays; this absence of testimony to the belief that Bacon wrote them is strong evidence that such belief did not exist until recently, a fact that tends to discredit the Baconian theory of authorship. The fact that in the writings of Dickens and Thackeray no mention is made of the bicycle is negative evidence that the bicycle had not then come into use. That Moses nowhere in his writings speaks of life after death is negative evidence that the Hebrews did not believe in the immortality of the soul. If admittedly capable and impartial officials do not inflict penalties for foul playing during a football game, there is strong presumption that little or no foul playing occurred.
The following paragraph, taken from a current magazine, shows how this kind of evidence may be handled very effectively:—
A sharp controversy has been raging in the European press over the question whether Gambetta secretly visited Bismarck in 1878. Francis Laur, Gambetta's literary executor, has published an article asserting that he did, and giving details (rather vague, it must be admitted) of the conversation between the two statesmen. But he offers not a scrap of documentary proof. He is not even sure whether the interview took place at Friedrichsruh or at Varzin. This is rather disconcerting, especially in view of the fact that Bismarck never made the slightest reference in his reminiscences or letters to the visit of Gambetta, if it occurred, and that the minute Busch never mentioned it. [Footnote: The Nation, September 5, 1907]