Saturday, March 27, 2010

Legal Matters: Evidence (3)


This is properly a rule of substantive law which is, in effect, that the terms of a contract or other legal instrument should be deemed to be embodied wholly in the written instrument executed by the parties thereto, or, stated in terms of evidence, the rule is that parol or oral evidence shall not be introduced for the purpose of varying the terms of a written instrument. The rule, though necessarily subject to many exceptions, is founded upon the just notion that when parties have deliberately embodied their agreement or transact in writing, they should not thereafter be allowed to dispute its terms. The following are the most important cases in which to vary the express terms of a written instrument testimony may be given:

a) Where the parties did not intend to reduce all the terms of the agreement to writing.

(b) When the writing or agreement is varied by a subsequent parol agreement.

(c) When the evidence is introduced to show that the written instrument has never takes effect because of the non-occurrence of some agreed condition precedent.

(d) When a term of the instrument is ambiguous and parol evidence is necessary to explain the meaning, and upon analogous grounds where the term of a written instrument has a technical or local meaning requiring oral explanation.

(e) When the proof of a custom which is in law a part of a contract or other document varies the effect or meaning of the written language.

(f) When in equity an action is brought to reform or rescind a written instrument, or construe a conveyance as a mortgage.


In general witnesses are allowed to testify only as to facts, and not as to their inferences or opinions based upon facts within their knowledge. To permit the witness to indulge in opinion testimony would be a usurpation of the function of the jury, whose duty it is to draw inferences of fact and to form an opinion, where an opinion is necessary to the verdict. Thus the witness, when the facts of a conversation are in issue, must testify as to the terms of the conversation and not his conclusions as to its meaning. There are, however, three important exceptions to the rule that opinion evidence is inadmissible. They are:

(a) Matters of common experience;

matters of common knowledge to a certain extent the result of inference. Thus, to testify that a certain day was cold, or that a knife was sharp, involves the operation of the witness's mind in drawing a conclusion; but since these are matters of common experience about which the conclusions of the witness are as trustworthy as those of a jury, such testimony is legally admissible as evidence.

(b) Matters not of common experience:

but about which the opinion of the witness is under the circumstances more trustworthy than any which could be formed by the jury. Thus, a witness may be so situated with reference to an event or combination of circumstances as to be able to draw a more accurate conclusion from them than the jury, which should rely wholly upon verbal testimony about the occurrence. Thus, the witness may be allowed to give his opinion of the distance between an approaching street-car and a pedestrian before warning of the approach was given; or, under certain circumstances, he may be allowed to give his opinion of the rate of speed at which the car was moving. His presence at the time of the event enables him to form a more accurate opinion than the jury, which can only rely upon a necessarily imperfect description of the occurrence.

(c) Expert Testimony:

A witness may be allowed to testify as to his opinion because, by reason of experience or special study and investigation, he is better qualified to form an opinion than the jury. Thus, physicians, engineers, handwriting experts, etc., are allowed to give opinion evidence in order to aid the jury in reaching a correct conclusion. They are not allowed, however, to express any opinion as to the truth or untruth of other evidence submitted to the jury, that being a matter of which the jury is qualified to judge. The testimony of experts, so far as it is opinion evidence is based upon the evidence already before the jury, assuming it or parts of it to be true. For that reason questions asked of expert witnesses are usually required to be hypothetical in form.

To be continued: Evidence (4)

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