Direct evidence, as a rule, is considered more valuable than indirect, but each kind is frequently sufficient to induce belief. The best possible kind of evidence, the kind that is least liable to contain error or falsehood, is a combination of both direct and indirect. Either one by itself may be untrustworthy. The unreliability of evidence given by eyewitnesses is shown by the conflicting stories they frequently tell concerning the same incident even when they are honestly attempting to relate the facts as they occurred. Also, it is always possible that the inferences drawn from a combination of circumstances may be entirely wrong. When, however, both kinds of evidence are available, each confirming the other and leading up to the same conclusion, then the possibility of error is reduced to a minimum.
The opportunity of the college student for obtaining evidence in his argumentative work is limited. A lawyer before entering upon an important case often spends weeks and months in investigation; scientists sometimes devote a whole lifetime in trying to establish a single hypothesis. But the college student in preparing an argument must obtain his evidence in a few days. There are several sources at his disposal. The first available source is his fund of general knowledge and experience. If a man can establish a statement by saying that he personally knows it to be true, he has valuable proof. Then the people with whom the student comes in contact constitute another source of evidence. Anyone who can give information on a subject that is being investigated is a valuable witness. Especially in discussions on questions which pertain to college life, the opinions and experiences of college men and of prominent educators are unsurpassed as evidence. But the greatest source of evidence for the student of argumentation is the library. Here he may consult the best thought of all time in every branch of activity. He may review the opinions of statesmen, economists, educators, and scientists, and introduce as evidence their experiences and the results of their investigations. Here he may familiarize himself with the current events of the world, and draw his own conclusions as to their significance. In fact, a well equipped library treats of all subjects, however broad or narrow they may be, and furnishes evidence for all sorts of debatable questions.
As not all evidence is equally valuable, a large part of the work of argumentation consists in applying tests to the evidence at hand for the sake of determining what facts are irrefutable, what are doubtful, and what are worthless. Moreover, one engaged in argumentation must test not only his own evidence but also that of the other side. No better method of refuting an opponent's argument exists than to show that the facts on which it rests are untrustworthy. Tests of evidence may be divided into two classes: tests of the source from which it comes, and tests of the quality of the evidence itself.
A. TESTS OF THE SOURCE OF EVIDENCE.
Since in courts of law, in college debate, and in all kinds of argumentation, facts are established by the testimony of witnesses, the sources of evidence are the witnesses who give it. The debater and the argumentative writer have not the opportunity, as has the lawyer, of producing the witnesses and permitting them to tell their own stories to the audience. He must himself relate the evidence; and, in order that it may be believed, he must tell whence it comes. The sources of evidence may be common rumor, newspapers, magazines, official documents, private citizens, or public officials. The extent to which these witnesses are accepted as trustworthy by the people before whom they are quoted determines in a large measure whether or not the evidence will be believed. Tests for determining the trustworthiness of witnesses will next be given.
The first test of the source of evidence should be:—
(1) Is the witness competent to give a trustworthy account of the matter under consideration?
To answer this question, first determine whether the facts to be established are such that any ordinary person can speak concerning them with reasonable accuracy, or whether they can be understood only by persons who have received special training. A landsman could well testify that a naval battle had occurred, but only a man with nautical training could accurately describe the maneuvers of the ships and tell just how the engagement progressed. A coal heaver's description of a surgical operation would establish nothing, except perhaps the identity of the people and a few other general matters; only a person with a medical education could accurately describe the procedure. The testimony of any one but a naturalist would not even tend to prove the existence of an hitherto unknown species of animal life. A witness without technical knowledge cannot give reliable evidence on matters of a technical nature.
Then, if it is found that the witness does possess the necessary technical training, or that no previous training is necessary, still further test his ability to give reliable evidence by asking whether he has had ample opportunity for investigating the facts to the existence of which he testifies. For even a skilled player sitting in the first base bleachers at a baseball game to criticise an umpire's decisions on balls and strikes is absurd; the opinion of a transient visitor to Panama on the methods used in digging the canal is not valuable; a traveler who has spent a single month in Japan cannot draw reliable conclusions on the merits and defects of its political structure. In not one of these cases has the opportunity for investigation been sufficient to render the witness able to give reliable evidence.
A current magazine in discussing the weakness of testimony that comes from incompetent witnesses says:—