The above are examples of the principal presumptions of law which arise when a particular state of facts has been established. But an exhaustive treatment of the subject is beyond the limits of this work.

It has been remarked, that it is not very easy to distinguish those presumptions which are binding on a jury from those which they are at liberty to disregard and to negative by their verdict, even when not rebutted.1 It is also difficult to draw the precise line between conclusive and disputable presumptions; between those which operate in the nature of estoppel, and those which are merely primâ facie and rebuttable in character. The solution of the difficulty will be found in a clear comprehension of the law of estoppel. In Jayne v. Price,2 Heath, J., said: "Nothing can be clearer than this: a presumption may be rebutted by a contrary and stronger presumption;" and it was decided in that case, that although proof of possession of land and receipt of rents is primâ facie evidence of a seisin in fee, yet proof of forty years' subsequent possession by a daughter, while a son and heir lived near and knew the fact, is much stronger evidence that the first possessor had only a particular estate.

1 Rosc. N. P. 24.

25 Taunt. 326.

61

CHAPTER VI.

ON EVIDENCE IN MATTERS OF OPINION.

SINCE it is the province of the judge or of the jury, according to the principles of presumptive evidence, to draw all inferences from facts, it follows that

A witness must only state facts: and his mere personal opinion is not evidence.

The object of this rule is to keep the witness, as much as possible, from trespassing on the functions of either judge or jury; and it is relaxed as often as an opinion of a witness can be regarded in the nature of a presumptive fact. Thus, in cases of insanity a medical witness cannot be asked whether he considers that the patient was insane; for that is the issue for the court and jury; but he may be asked whether certain symptoms are indications of insanity, and his answers are evidence for the guidance of the court and jury.1 Thus, on an issue as to the sanity of a testator, Sir F. Thesiger, for the defendant, tendered a letter (purporting to be from the testator) to a medical witness, and proposed to ask him whether the writer of such a letter could be of sound mind. Martin, B., held that this could not be done; but that the letter must first be proved to be in the testator's writing, and that the witness might then be asked if it was a rational letter.2,

1 R. v. M'Naghten, 10 Cl. & Fin. 200.

2 Sharpe v. Macaulay, Western Circuit, 1856, MS.

Accordingly, in actions of slander, where it is important to prove an innuendo and that the obvious and natural meaning of a word was not that which the speaker intended to convey to the witness, the witness cannot be asked what he understood by the language; for the answer to such a question would be in the nature of an inference and a mere personal opinion. But questions ought to be put to him which tend to elicit all the surrounding facts and circumstances which led him to understand the words in a slanderous sense; and he may be asked whether there was irony in the speaker's tone at the time, and generally whether there was anything to prevent him from understanding the words in their ordinary sense.1

In the leading case of Carter v. Boehm,2 it was a question whether a policy of insurance was vitiated by the concealment of facts which had not been communicated to the underwriters. A broker gave evidence of the materiality of the facts, and stated his opinion, that if they had been disclosed the policy would not have been underwritten; but the court held his statement to be inadmissible. Lord Mansfield said: "Great stress was laid upon the opinion of the broker; but we all think the jury ought not to pay the least regard to it. It is mere opinion, which is not evidence; it is opinion after an event; it is opinion without the least foundation from any previous precedent or usage; it is an opinion which, if rightly formed, could be drawn only from the same premises from which the court and jury were to determine the cause, and, therefore, it is improper and irrelevant in the mouth of a witness."

This judgment of Lord Mansfield contains the principles on which mere opinion is not received as evidence. But it is right to state that his view of the law, as to this particular case, has been much controverted; and that it has been considered by other learned authorities,3

1 Davies v. Hartley, 3 Exch. 200; Barnett v. Allen, 27 L. J. 412, Ex.; 31 L. T. Rep. 217.

2 3 Burr. 1905; 1 Sm. L. C. 270.

3 See note to Carter v. Boehm, 1 Sm. L. C.

to come within the exception to the rule which is now to be stated; for it is held that

The opinions of skilled or scientific witnesses are admissible evidence to elucidate matters which are of a strictly professional or scientific character.

In Campbell v. Richards,' Lord Denman, C. J., said:

"Witnesses conversant in a particular trade may be allowed to speak to a prevailing practice in that trade; scientific persons may give their opinions on matters of science; but witnesses are not receivable to state their views on matters of legal or moral obligation, nor on the manner in which others would probably be influenced if the parties had acted in one way rather than another. In the great case of Carter v. Boehm, a broker, who was called as a witness for the plaintiff, stated on cross-examination, that in his opinion certain letters ought to have been disclosed, and that if they had, the policy would not have been underwritten. The jury, however, found, against the witness's opinion, a verdict for the plaintiff. When his opinion was pressed, as a ground for a new trial, Lord Mansfield, in the name of the whole court, declared that the jury ought not to pay the least regard to it, because it was mere opinion and not evidence. The same doctrine is laid down in a case of Durrell v. Bederley, by Gibbs, C. J., though he received the evidence on great pressure. He said, 'The opinion of the underwriters on the materiality of facts and the effect they would have had upon the premium, is not admissible in evidence.' Lord Mansfield and Lord Kenyon discountenanced this evidence of opinion, and I think it ought not to be received. It is the province of a jury and not of individual underwriters to decide that facts ought to be communicated. It is not a question of

15 B. & Ad. 846.

science, in which scientific men will mostly think alike, but a question of opinion, liable to be governed by fancy, and in which the diversity might be endless. Such evidence leads to nothing satisfactory and ought to be rejected."

It will appear from this judgment that the two rules, as stated above, are recognised and acted on universally; and that the only practical difficulty in applying them, exists in the question as to what is and what is not a subject of scientific inquiry. The inclination of modern authorities appears to be to enlarge the definition; and it is probable that if Carter v. Boehm, and Campbell v. Richards, were to be decided again, it would be held that the nature of mercantile traffic, and the principles of insurance in particular, are sufficiently recondite to entitle them to the privilege which was disallowed in those cases.1 And in Greville v. Chapman,2 which was an action for libel arising out of a racehorse transaction, it was held by Lord Denman himself, that a member of the Jockey-club might be asked as a witness, whether he did not consider a certain course of conduct to be dishonourable.

Books are admissible to show the sense in which words are used; and especially in cases of libel defendants have been permitted to refer largely to previous publications, and to read them as part of their defence, in order to show that certain forms of expression were not meant as matter of reproach or ridicule; and to explain whether they have been used in a metaphorical or literal sense. Books also may be used to show the opinions of their writers on their subjects; but such opinions cannot be made evidence of specific facts. "Thus, it is not competent in an action for not farming according to covenant, to refer to books for the purpose of showing what is the best way of farming. Nor in an action on the warranty of a horse would it be allowable to refer

11 Sm. L C. 286 a., Rickards v. Murdock, 10 B. & C. 527. 25 Q. B. 731.