Practical Argumentation · Chapter 9I. Evidence
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There are two kinds of evidence: (a) direct, and (b) indirect or circumstantial. If a man sees a gang of strikers set fire to the buildings of their former employer, his evidence is direct. If, however, he only sees them stealthily leaving the buildings just before the fire breaks out, his evidence is indirect. In the latter case the man's testimony is direct evidence that the men were in the vicinity of the fire when it started, but it is indirect evidence that they perpetrated the crime. If a student who has failed to do good work throughout the term, and who has had little or no opportunity for special preparation, passes in a perfect paper at the close of an examination, the presumption is that he has received aid. The evidence on which this supposition rests is entirely circumstantial. But if some one saw the student obtaining aid, that fact would be direct evidence against him.
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.