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84 HARVARD LAW REVIEW. consider whether to believe his statements or not; if to other facts, of an evidential sort, then the jury were to judge of their import and their tendency. The witness was not to say that he " thought " or " beheved " so and so ; it was for the jury to state what they thought and believed. The witness must say what he had " seen and heard ; " he was an " oyant et veyant!' But then, simple as all this sounds, the distinction could not serve in many nice and critical inquiries. In the loose and easy administration of the law of trials that existed as long as juries went on their own knowl- edge, and needed no witnesses or evidence at all, and at a time when, even if they had witnesses, they were at liberty to disregard them and to follow their own personal information, it was possible to get along without nice discriminations ; so that the law of evi- dence had hardly any development at all until within the last two centuries ; and it was but slight before the present century. In a sense all testimony to matter of fact is opinion evidence; i. e., it is a conclusion formed from certain phenomena and certain mental impressions. Yet that is not the way we talk in courts or in com- mon Hfe. Where shall the line be drawn? When does matter of fact first become matter of opinion? A difficult question. But some things are clear. There are questions which require special training and knowledge to answer them. A jury, unless it be one of experts, and, as such, ill adapted, perhaps, for the general purposes of trials, cannot deal with them. On such questions, then, the ordinary jury may be assisted by skilled witnesses, who give their opinions. There are other questions, not requiring skill or training, but only special opportunities of observation, like handwriting, and the value of property, on which opinions of ordinary witnesses having such opportunities may be given. How far does this go? There is much apparent perplexity in the cases. In a very great degree it results from differences of practical judgment in applying an admitted rule, — the admitted rule being that opinion evidence is not generally receivable, and the difference arising from differing judgments as to what is and is not really to be called opinion evi- dence in the sense of the rule. It has been said, judicially, that " there is, in truth, no general rule requiring the rejection of opin- ions as evidence." ^ Without acceding quite literally to that, there is ground for saying that, in the main, any rule excluding opinion evidence is limited to cases where, in the judgment of the court, it 1 Hardy v. Merrill, 56 N. H. 227, 241.