Whenever the subject matter of a legal enquiry is such that, from its partaking of the nature of a science, art or trade, inexperienced persons are unlikely to prove capable of forming a correct judgment upon it without assistance, then the opinions of witnesses possessing peculiar skill and knowledge in the matters in question are admissible in courts of justice. And it is only when the matter inquired of lies within the range of the peculiar skill and experience of the witnesses, and is one of which the ordinary knowledge and experience of mankind does not enable them to see what inference should be drawn from the facts, that the skilled witnesses may supply opinions as their guide (267). The rule admitting the opinions of experts in such cases is founded on necessity, for juries are not selected with any view to their knowledge of a particular science, art or trade, requiring a course of previous study, experience or preparation (268).

The rule of law on which the giving in evidence the opinion of witnesses, who know nothing of the actual facts of the case, is founded, is not peculiar to medical testimony, but is as a general rule applicable to all cases where the question is one depending on skill and science in any particular department. ... In general it is the opinion of the jury which is to govern, and this is to be formed upon the "109" proof of the facts laid before them. But some questions lie beyond the scope of the observation and experience of men in general, yet are quite within the observation and experience of those whose peculiar pursuits and profession have brought that class of facts frequently and ha­bit­ually under their con­sid­er­a­tion. When, therefore, a question arises in a court of justice upon any such subject, and certain facts are proved by other witnesses, one skilled in such subject may be asked his opinion as to the character of such facts; or he may be asked his opinions on certain facts observed by himself. This is true with regard to any question of science, because persons conversant with such science have peculiar means, from a larger and more exact observation and long experience in such department of science, of drawing correct inferences from certain facts observed by themselves or testified to by other witnesses. The opinion of such witnesses is designed to aid the judgment of the jury in regard to the influence and effects of certain facts which lie out of the observation and experience of persons in general (269). These witnesses are called “experts.” This term seems to imply both superior knowledge and practical experience in the art or profession. But generally nothing more is required to entitle one to give testimony as an expert, than that he has been educated in the particular art or profession; for persons are presumed to understand questions pertaining to their own profession or business (270).

The practice of admitting the evidence of experts is an old one: in the Roman Law they are frequently alluded to, and in the earliest Common Law reports they are spoken of as of established usage. Says Saunders, J., “and first I grant that if matters arise in our law which "110" concern other sciences or faculties we commonly apply for the aid of that science or faculty which it concerns. In a case of mayhem the defendant prayed the court that the wound might be examined, on which a writ was issued to the sheriff to cause to come “medicos chirurgos de melioribus London. ad informandum Dominum regem et curiam de his quÆ eis exparte Domini Regis injungerentur (271).”

Some Judges and writers have very little respect for the evidence and opinions of experts. An Iowa Judge says, observation and experience “teach that the evidence of experts is of the very lowest order, and of the most unsatisfactory kind.” One from Maine, speaks of “the vain babblings and oppositions of science so called, which swell the record of the testimony of experts when the hopes of a party depend rather upon mystification than enlightenment.” An Illinois Judge quotes a distinguished occupant of the bench as saying, “if there was any kind of testimony not only of no value, but even worse than that, it was in his judgment that of medical experts.” Lord Campbell said, “Hardly any weight is to be given to the evidence of what are called scientific witnesses: they come with a bias on their minds to support the cause in which they are embarked” (272). Taylor says, “Perhaps the testimony which least deserves credit with a jury is that of skilled witnesses. ... Being zealous partisans their belief becomes synonymous with faith as defined by the apostle, and it too often is but the substance of things hoped for, the evidence of things not seen” (273). On the other hand, Best says, “It would not be easy to overrate the value of the evidence given in many difficult and delicate enquiries, not only by medical men and physiologists, but by learned "111" and experienced persons in various branches of science, art and trade” (274). And many Judges have spoken of the essential aid to courts and juries rendered by the opinion of the experienced, skilful and scientific witness who has a competent knowledge of the facts involved.

When one takes his place as an expert before a court, a legal paradox is instituted on his behalf, by which he is allowed to testify—not as to what he knows, but to what he believes or forms an opinion upon, based necessarily on probabilities of analogy as well as experience. Nothing is required (in the absence of any statutory provision to the contrary) to entitle any one to give evidence as a medical witness, than that he has been educated in the science of medicine; and this he may be by study without practice, or by practice without study; it is not necessary that he should be a physician, or have studied for one, nor be a graduate, nor one licensed to practise, nor need he be or have been a practitioner (275). One may be competent to testify as an expert, although his special knowledge of the particular subject of enquiry has been derived from the reading and study of standard authorities, and not from experience or actual observation. But one cannot qualify himself as an expert in a particular case merely by devoting himself to the study of authorities for the purposes of that case, when such reading and study is not in the line of his special calling or profession and is entered upon to enable him to testify in the case. In Vermont, however, it has been held that mere education as a physician, without some practice as such, is insufficient to qualify one as an expert; and in Arkansas, it is said, that competency must be shewn from study and experience. In New York, it has been held that one otherwise qualified, who is "112" a physician and surgeon, may give evidence, although not in full practice at the time; this fact merely goes to affect his credit (276).

It is not necessary that the physician should have made the particular disease involved in the enquiry a specialty; medical men of practice and experience are experts, and their opinions are admissible in evidence upon questions that are strictly and legitimately embraced in their profession and practice. If one has made the matter in question a specialty, doubtless his opinion will be of more value than if he has not; and it has been said, that one who has devoted himself exclusively to one branch of his profession cannot give evidence as an expert on another (277). For example, one not an oculist may speak as to the cause of injuries to an eye; one who has not made diseases of the mind a special study may give his opinion as to the existence of insanity; one not a practical chemist or analyst, but understanding the practical details of chemistry and the means of detecting poisons, may testify as to the tests in the chemical analysis of a stomach, and as to the tests usually applied to detect poison (278). The law will even allow a physician to speak as to the length of time a mule has been suffering from a disease (279). But one who has had no experience as to the effect upon health of illuminating gas cannot testify in relation thereto as an expert (280). Nor can one who has for thirty years been exclusively treating the insane be permitted to testify, as an expert, on "113" the mental capacity of a person in the last stages of disease, who has not been previously insane (281).

To render the opinion of a witness competent evidence, he must, in general, be in some way peculiarly qualified to speak on the subject, and have knowledge not possessed by the mass of persons of ordinary experience and intelligence (282). Upon this principle, a priest who had studied physiology and psychology, in order that he might pass upon the mental conditions of communicants in his church, and who had so to decide daily, was permitted to speak as to the mental state of a woman whom he had attended in her last illness (283).

It is a question of fact to be decided at the trial, by the Court, whether a witness offered as an expert has the necessary qualification (284). And the matter cannot be referred to the decision of the jury. The decision of the Judge at the trial will not be interfered with by the Court, except in a clear and strong case (285).

Any one offered as an expert who cannot establish the fact of special knowledge or skill, in the particular department which he is called upon to illuminate, will be rejected. A Court before permitting an expert to testify may examine him, or hear evidence, to satisfy itself that the witness is really what he assumes to be (286). "114"

“We find no test laid down,” says the Supreme Court of Indiana, “by which we can determine with mathematical precision just how much experience a witness must have had, how expert, in short, he must be, to render him competent to testify as an expert.” But it is for the Court to decide, within the limits of a fair discretion, whether the experience of the proposed expert has been such as to make his opinions of any value; mere opportunities for special observation will not be deemed sufficient (287).

While the Court, or Judge, determines the competency of the witness to testify as an expert, the weight to be accorded to his testimony is for the jury to decide. The testimony of an expert is to be weighed and tested like any other kind of evidence, and is to receive just such credit as the jury may think it entitled to. It is intended to enlighten their minds, not control their judgment (288). The jury are not bound by the opinions of medical experts: they may weigh their opinions like any other evidence. They may act against the greater number of opinions and in favour of the fewer; for the opinion of one expert may, on account of his greater knowledge and experience on the subject, or from his giving further details of the case, or more probable reasons for his opinions, be of greater value to the jury than the opposite opinions of several (289).

Ordronaux holds that a physician, although confessedly possessing the ordinary experience of his profession, may quoad some particular problem in medical science not be an expert in the best and most critical sense of the term. Non omnes omnia possumus. Once received as an expert, "115" the maxim “Cuilibet in sua arte perito credendum est,” must be applied, and he cannot be contradicted by any unskilled person (290).

In 1869, the Chief Justice of the Kentucky Court of Appeal well said, that “the opinions of experts not founded on science, but on a mere theory of morals or ethics, whether given by professional or unprofessional men, are wholly inadmissible as evidence.” Hence the opinion of even physicians that no sane man in a Christian country would commit suicide, not being founded on the science or phenomena of the mind, but rather a theory of morals, religion and future responsibility, is not evidence (291).

In the matter of expert testimony, as in other matters, the law does not recognize any particular school of medicine to the exclusion of others. The popular axiom that doctors differ is as true now as ever it was, and so long as it continues to be so, it is impossible for the law to recognize any class of practitioners, or the followers of any particular system, or method of treatment, as exclusively entitled to be regarded as “doctors” (292).

The physician called to give evidence as an expert should understand at the outset that he is not called to express any opinion upon the merits of the case, but only on some questions of science raised by the facts proved; that he has no concern in the issue of the trial, and that whichever side calls him he is in no wise the witness—much less the advocate—of that side. He is truly an adviser of the Court, an amicus curiÆ, rather than a party interested in the result of the trial. Balbus in his commentaries on the code says, “Medici proprie non sunt testes, sed est magis judicium quam testimonium.” Experts, no matter on what "116" they testify, simply supply data, as to whose competency, relevancy and weight, the Court is to judge, and as to which the Court is finally to declare the law. Where the facts testified to by experts are undisputed, and when they are the results of a particular science or art, with which such experts are familiar, then the Court accepts such facts, and declares the law that therefrom springs; where the facts are disputed then the jury is to determine where the preponderance of proof lies. But when the testimony of the expert touches either law or speculation, psychology or ethics, then such testimony is to be received as mere argument, which if admissible at all is to be treated simply as if addressed to the judgment of the Court (293).

In his examination in chief an expert may not only give his opinion itself, but also the grounds and reasons of it; in fact it has been held that it is his duty to state the reasons of his opinion and the facts on which it is based, and if it is not sustained by them it is entitled to little weight (294).

The opinion of a medical man is admissible upon, the condition of the human system at any given time; the nature and symptoms of disease; the nature and effects of wounds; the cause of death; the cause or effect of an injury; the character of the instrument with which a wound was produced; the effect of a particular course of treatment; the likelihood of recovery; the mental condition of a person; and on similar subjects. For instance, where one was indicted for endeavouring to procure abortion, the opinion that the woman was pregnant at the time is relevant (295). Where the question was whether a certain "117" blow was sufficient to cause death; or whether a wound and fracture on the head was caused by a fall; or whether the fractures of the skull were caused by a gun; or whether a gun-shot wound caused death; the opinions of physicians were held admissible (296). The opinion of medical experts will be received upon the question as to whether an abortion has been performed, or whether certain drugs are abortives, or certain instruments adapted to produce an abortion (297). Experts may testify, after having made a chemical analysis of the contents of the stomach, as to the presence of poison in the body; and, without such analysis of a mixture, a chemist may speak of its ingredients (298). Those accustomed to make chemical and microscopic examinations of blood and blood stains may speak as to whether certain stains are made by human or other blood. So, too, they may speak as to the ink in questions as to handwriting (299). So, too, they may be asked their opinions touching the permanency of any injury forming the subject of an action. Also, in an action for damages against a railway company, a physician may be asked at what period after the injury the plaintiff would be most likely to improve, if he were going to recover at all (300). Where Barber sued Meriam for injury to his wife, and she had been treated professionally for some weeks by Dr. H., the opinion of another physician as to the effect of Dr. H.’s treatment was considered "118" admissible (301). And so in a case of malpractice a medical man may be asked whether the practice pursued was good practice (302). He may be asked as to the nature and properties of the medicines employed by another physician in the case in question; also, as to the practice with regard to consultations; also, whether, in his opinion, a patient’s death was or was not the result of neglect or want of skill on the part of the attending physician (303). But he cannot be asked his opinion as to the general skill of the physician on trial; nor the general reputation of the school which the doctor in trouble attended; nor can he say whether, from all the evidence in the case, the defendant was guilty of malpractice, for that is the question for the jury; nor can he say whether a physician has honorably and faithfully discharged his duty to his professional brethren (304).

It has been held that a medical witness may give his opinion upon new and hitherto unknown cases whenever he swears that he can form such an opinion, even though at the same time he should admit that precisely such a case had never before fallen under his observation, nor under his notice in the books. The man of science is distinguished from the empiric in nothing more than in not relying on specifics, and also not waiting for the exact similitudes in things material and immaterial before forming a judgment as to their similarity (305).

It must always be remembered that medical men, when called as skilled witnesses, may only say what, in their judgment, would be the result of certain facts submitted to their consideration, and may not give an opinion as to "119" the general merits of the case, nor on the very point which the jury has to determine, nor on things with which a jury may be supposed to be equally well acquainted (306).

As a recent writer puts it, a medical man cannot testify as to matters not of skill in his profession, nor conclusions, nor inferences which it is the duty of the jury to draw for themselves. For instance, it was held that in a trial for murder the opinions of the surgeons as to the probable position of the deceased, when he received the blows which caused his death, are incompetent. The Judge said that he was not aware that surgeons were experts in the manner of giving blows of the description in question, or determining how the head must be placed so as most conveniently to receive them (307). Whenever the subject matter of the enquiry is of such a character that it may be presumed to lie within the common experience of all men of common education, moving in ordinary walks of life, the rule is that the opinions of experts are inadmissible, as the jury are supposed—in all such matters—to be entirely competent to draw the necessary inferences from the facts spoken of by the witnesses (308). Nor was the opinion of a medical witness admitted where the question, in an action for libel, was whether a physician in refusing to consult with the plaintiff had honorably and faithfully discharged his duty to the medical profession. The Judge said, the jury having all the facts before them were as capable of forming a judgment upon that point as the witness himself. Nor can an expert give an opinion of the opinion of another expert (309). A medical man is considered an expert on the subject of the "120" value of medical services (310). But he is not so considered when the question is one as to the amount of damages for a breach of contract not to practise physic in a certain town (311).

The rule as to excluding experts from the court room during the examination of witnesses has been laid down, in England, thus: “Medical or other professional witnesses, who are summoned to give scientific opinions upon the circumstances of the case, as established by other testimony, will be permitted to remain in court until this particular class of evidence commences; but then, like ordinary witnesses, they will have to withdraw, and to come in one by one, so as to undergo a separate examination.” And in the United States the principle is similarly stated (312).