How have the courts considered opinion in the context of an action for negligence?

they had made such subject a previous study. Again, the market value of an article at a given time, upon the allowance of damages on which a jury has to pass, is frequently a question such as dealers in that article can alone decide. There it is a matter of necessity to call in the experienced or instructed opinion of such witnesses. No proof of the naked state of facts as to a ship after a storm could perhaps enable a landsman, however in telligent, to judge of those necessities which are so often to be inquired into in marine contracts. Thus, also, in an action for negligently steering a ship, as in Malton v. Nesbit, 1 Car. & P. 70, mere proof of the naked facts could not enable a jury of landsmen to draw any inference; and experienced nautical men are called in to prove whether facts of that kind amount to unjustifiable negligence. Opinion is admitted when a jury is incompetent to infer, without the aid of greater skill than their own, as to the probable existence of the facts to be ascertained, or the likelihood of their occurring from the facts actually proved before them. Indeed, it would be more logically accurate to say that mere opinions, even of men, professional or expert, is not admissible as such; but that, facts having been proved, men skilled in such matters may be admitted to prove the existence of mere general facts or laws of nature, or the course of business, as the [118 N.Y. 437]case may be, so as to enable the jury to form an inference for themselves. Thus, the existence of certain appearances in the dead body having been proved, the chemist testifies that such appearances invariably or generally indicate the operation of some powerful chemical agent. His scientific opinion is, in fact, his testimony to a law of nature. All these are testimonies to general facts which the jury can ascertain in no other way, and which, when proved, afford them the means of drawing their own conclusions from the whole mass of testimony taken together. The same reason of absolute necessity has compelled the admission of evidence of opinion in certain cases where the poverty of human language makes it absolutely impossible to separate in words the minute and transient facts observed by the witnesses from the inference as to some other fact irresistibly connected with the former in his own mind. Testimony as to handwriting, I think, resolves itself into this; as no words can freely convey to others the minute particularities on which such judgment is founded. So, too, in questions of identity as to men, to goods, horses, etc., though the facts on which judgment is founded may be partially stated, still the judgment or opinion is admitted. In these cases the witness swears as to the present conviction in his own mind as to an actual fact, though deduced from circumstances which cannot be made palpable to others. It is often difficult to draw a line of distinction between testimony to simple facts and the statement of such immediate and conclusive inferences as the witness forms in his own mind. And hence general physical facts or truths not known to the common mind, and not apprehensible by the senses at single or casual observation, may be shown by one who has knowledge or skill or experience,-an expert,-to aid the jury in the ascertainment of the truth.