What is the test for admissible evidence that contradicts the terms of a written contract?

All of the evidence offered by the defendant about a completion date, or dates, is offered for the express purpose of contradicting the terms of a written instrument. That evidence is not admissible. That is a proposition which has had a long history in the law. In Goss v. Nugent,[1] the classic statement of the rule was expressed in these words: By the general rules of the common law, if there be a contract which has been reduced into writing, verbal evidence is not allowed to be given of what passed between the parties, either before the written instrument was made, or during the time that it was in a state of preparation, so as to add to or subtract from, or in any manner to vary or qualify the written contract ...[2]

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.