An Agreement Reduced to Writing Deemed the Whole

When the terms of an agreement have been reduced to writing by the parties, it is to be considered as containing all those terms, and therefore there can be between the parties and their representatives, or successors in interest, no evidence of the terms of the agreement other than the contents of the writing, except in the following cases: 1. Where a mistake or imperfection of the writing is put in issue by the pleadings; or 2. Where the validity of the agreement is the fact in dispute. But this Section does not exclude other evidence of the circumstances under which the agreement was made or to which it relates, as defined in § 2515 [Circumstances to be Considered], or to explain an extrinsic ambiguity, or to establish illegality or fraud. The term agreement includes deeds and wills, as well as contracts between parties.