234 HARVARD LAW REVIEW. Chief Justice Shaw, 1 in deciding that an expert may state the grounds on which he founds his opinions, defines the extent of the doctrine quite clearly, as follows : " The ground on which an expert, a person of large experience in any particular department of art, business, or science, is permitted to testify to his opinion, is, that from his larger experience and more exact observation of facts, and the connection between certain appearances, and their causes or results, he is able to draw correct conclusions from cir- cumstances, which a man of ordinary knowledge and experience could not do. The circumstances on which such an opinion may be founded are either facts of general notoriety, assumed to be known to all persons of skill and experience in the department to which they pertain, and which, when explained, may be compre- hended and applied by any person of good understanding; or it may be founded on facts proved. " Such general facts, assumed to be generally known without specific proof, because they are capable of being known and understood, without any such proof, to all inquirers, are vastly too numerous to define ; but the point may be illustrated by saying they are such as the elements and forces of physical nature, the structure, capacities, and functions of the human and other animal bodies, the common powers, propensities, and passions of human nature, and the impelling and governing motives to human action. As these are capable of being com- prehended, when explained, without specific proof, it appears to the court that the witness should be permitted to explain the grounds and reasons of his opinion to the court and jury; they may readily perceive the force of his reasoning, the soundness or fallacy of his logic, and therefore judge of his capacity to give an opinion on the subject, and the correctness of his conclusions, and consequently the weight due his opinion." It was evident that in giving this decision Chief Justice Shaw never contemplated the possibility of a witness detailing facts which it would not be legitimate for the jury to consider as a ground for their verdict, and although other cases have been less definite in their language, the principle does not seem to have been extended by the decisions. The analogy of expert testimony would therefore seem to show just the contrary of the claim that irrelevant details may be stated 1 Dixon et al. v. Fitchburg, 13 Gray, 546, at p. 555.
