Then in the last place, if the statements are made as those of an expert, the value of his testimony is in the exact ratio of his experience in the particular field discussed. Here is the opportunity for a very common error in argument. Frank in debating might have urged the opinion of Professor A or Doctor B, and Jack in his reply urged confidently the opinions of Judge C or General D as disposing of the evidence of Frank’s witnesses. But they would simply offset one opinion with another unless Jack could show that the judge and the general knew more about the subject matter than the professor and the doctor. Character or standing or position would avail nothing. In discussing conservation, for example, the opinion of the most eminent professor in a theological school or the greatest expert in running electric cars would not be received in questions of soil moisture or timber culture. It is not the standing of the man generally; it is his knowledge of the subject discussed, which makes his opinions acceptable as evidence. The evidence of a farmer or hunter or trapper, although unable to read or write, might outweigh that of the so-called expert.
Generalizations.—Next, in weighing circumstantial evidence, you will observe that there are three common forms in which it appears. There is first the generalization—very frequently the shape an expert’s opinion takes. If he says that timber tracts are worth so many dollars an acre as timber and so many dollars an acre as farms, his opinion is valuable or not depending upon what the basis of his judgment is. If it develops that he has observed many hundreds of such tracts of land, and that these different tracts lay in many different places, you safely conclude that his testimony, although a generalization, is sound because it is based upon wide observation. If he has inspected but very few such tracts, you conclude that his generalization is unsound, and worthless in helping you prove the facts.
Do not hesitate to apply the probe to the qualifications of your opponent’s expert, but be equally sure you establish the right of your own witness to speak as such. For example, I remember a case where the question involved was whether dehorning cattle was cruel within the meaning of a statute forbidding cruelty to animals. One side brought forward as expert professional witnesses certain surgeons eminent in their profession. They testified that the horn of the cow contained just under the hard bony shell a thin membranous structure, the extension of the periosteum, which was very sensitive to pain. They testified that they had known of cases where the shell of the horn had been removed by accident and this membrane exposed, whereupon the animals had given every manifestation of suffering excruciating pain. They argued from the sensitiveness of the periosteum, through these few instances of accident, to the deduction that dehorning cows, that is, sawing the horn off at right angles to this membrane, must also be extremely painful and hence cruel.
Opposed to these expert opinions of these surgeons based upon these few cases, were the opinions of farmers who did not pretend to have the scientific knowledge of the surgeons but who had dehorned many cows. They testified that they had seen but slight evidences of pain, that the animals had shown annoyance rather than distress, and had generally at once gone to eating. The farmers had no titles or university degrees; the experts had both, and supposedly in the very field of the debate—for that was what it was, although in a court of law. The superior qualifications of the plain unlettered farmers made their evidence of much greater value as expert testimony than the deductions of the expert gentlemen from the city.
Suppose you are discussing the adoption of a law prohibiting child labor. You can quote with confidence the opinion of a social worker whose daily duties keep her among operatives in mills. By her daily observation she is in a position to know the evil effects of labor on immature bodies and minds. So is the doctor or the teacher who sees these children daily. On the other hand, the farmer, who, although honest in his deduction from the results of his own labor, is not qualified because he himself worked as a boy on a farm to speak understandingly of the effects of child labor in mills or factories. He must confine his testimony to the effect of child labor on the farm. Nor is the woman who knows nothing of labor on farm or in factory qualified to speak at all, sentimentally aroused as she may be, unless she can show observation of conditions herself or the intelligent study of the well founded opinions of others.
Cause and Effect.—Then again there is in circumstantial evidence the inference you draw from the relation of cause and effect. To use a very familiar example: Suppose you pass a piece of ground where to your knowledge there stood but a very few weeks before a heavy growth of timber and observe that the trees are gone. If you see piles of freshly sawed lumber, you can argue that the trees have been cut by man; if on the other hand you see bent and torn trees, you can as surely reason that a cyclone has visited that neighborhood. In each case you argue from effect back to cause. This seems very simple, but is often a very treacherous argument, for the alleged cause may be entirely inadequate or may be so involved with other unrecognized causes that the deduction you make is not warranted. You must examine the connection very carefully and make sure that it is a causal relation and not a casual one.
Argument from Resemblance.—Then there is the argument from resemblance, which is simply another form of causal relationship. The argument, for example, would run that conservation must surely be a good thing for the United States because it was a good thing for New Zealand. That conclusion is sound if the conditions in the two countries are the same, but not otherwise; and it is upon this rock of difference in conditions that arguments from resemblance so often split. Watch them. The same rule may not apply to reforestration in Idaho as in Maine, for instance, because conditions are dissimilar in many respects.
If you carefully study these forms of argument, and can estimate the value of opinions and circumstantial evidence, you will not only be able to build up a bomb proof argument of your own but you will be able to detect fallacies in your opponent’s discussion. When you have mastered the one branch of the case you are equipped to meet the other, for the two are identical.