The reason for the rule was explained in Ellis v. Abell,[3] in the following words: ... and it is well to bear in mind the reason of the rule, that when parties have deliberately put their engagements in writing in such language as imports a legal obligation it is only reasonable to presume that they have introduced into it every material term and circumstances; and consequently all parol testimony of conversations or declarations made by either of them, whether before or after or at the time of the completion of the contract, will be rejected, because such evidence, while deserving far less credit than the writing itself, would inevitably tend in many instances to substitute a new and different contract for the one really agreed upon.
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