Whosoever is skilled in his profession is to be believed.
Evidence of a fact relevant to the matter at issue between the parties, -within the personal knowledge of a witness, is allowed to be given as of right ; as, where the witness himself stated an account between the parties, paid a sum of money or delivered certain goods. But, the opinion of a witness upon a fact, or state of facts, is only received when it comes within the meaning of this maxim ; as, the opinion of a surgeon, architect, etc, upon questions relating to surgery, architecture, etc. So, where in an action the question was whether or not an embankment erected to prevent the overflowing of the sea had caused the choking up of the harbour, the opinions of scientific men as to the effect of such an embankment upon the harbour were held to be admissible. So a physician, though he may not have seen the patient, may, after hearing the evidence of others at the trial, be called upon to speak to the nature of the disease described by them ; as, whether or not the facts proved are symptoms of insanity ; but this opinion must not go to the fact that the patient is insane, but merely that the symptoms detailed by the witnesses are those of insanity. The opinion of insurance brokers as to whether the communication of certain facts would have varied the terms of the insurance, has been admitted in actions on the policy ; but not in matters of mere opinion only ; as where, in an action on a policy the opinion of the broker that, had certain letters been disclosed at the time of underwriting the policy, it would not have been underwritten, was sought to be given as evidence, this was held to be mere opinion and not evidence. Where the question is whether or not a seal has been forged, seal engravers may be called to show the difference between the impressions made by the original seal and those made by that supposed to be forged. So the opinion of a student of the law of a foreign country to prove that law, is inadmissible, as being opinion merely, he not being within this rule ; though .the opinion of a person versed in the laws of a foreign country is admissible. Evidence of handwriting lies between proof positive and scientific knowledge. Ancient M.S. documents may be proved by a witness expert in comparing writing by the same author ; but handwriting generally, must be proved by some person who has either seen the person write, or who has such an acquaintance with his writing, through correspondence acted upon or admitted, as leaves no doubt upon his mind that the writing in question is that of the party by whom it is said to have been written.
This maxim may be properly associated with that of “Ad qusestionem facti non respondent judicis, ad quaestionem juris non respondent juratores” — To questions of fact judges, and to questions of law the jury, do not answer. The judges, jury, and witnesses have each their special prerogative, but they cannot exceed its limits. The judges apply the law to the facts ; the jury judge the facts ; but even they cannot give an opinion without having facts whereon to found their judgment, the truth of which facts it is their special province to determine. The witnesses depose to the facts. Witnesses are, however, of two kinds — one deposing to the facts merely, and the other giving an opinion or judgment upon the facts for the information of the jury ; and these latter are called “ perita,” who give their opinion according to their skill in their profession in matters of art and science.
Co. Litt. 125; Folkes v. Chadd, 3 Doug. 157; Campbell v. Richards, 5 B. & Aid. 840 ; Durrell v. Bederley, Holt N. P. C. 285 ; The Sussex Peerage Case, 11 C. & F. 85 ; Baron de Bode v. Reg., 8 Q. B. 208 ; M'Naughten'a Case, 10 C. & F. 200 ; Chapman v. Walton, 10 Bing. 57 ; Bris- towe v. Sequeville, 5 Exch. 275 ; Tracy Peerage Case, 10 C. & F. 154 ; Chaurand v. Angerstein, Peake Ca. 44 ; Berthon v. Loughman, 2 Stark. 258 ; Doe v. Luckermore, 5 A. & E. 730.