What principle states that only written contract terms are valid?

The principle that states that only written contract terms are valid is known as the Parol Evidence Rule. This rule serves to maintain the integrity of written agreements by preventing parties from introducing extrinsic evidence—such as oral statements or written communications made prior to the signing of the contract—that would contradict or modify the written terms of the agreement.

By adhering to the Parol Evidence Rule, courts uphold the idea that a fully executed written contract represents the complete and final understanding of the parties involved. This principle ensures clarity and transparency when interpreting a contract, as it discourages disputes over what was said before the contract was signed.

Understanding this rule is crucial in legal contexts, especially when parties later attempt to argue about the meaning or agreements that were not included in the written document. The Parol Evidence Rule effectively promotes the reliance on documented agreements, thus providing stability and predictability in contractual relationships.