After a 30-year career as a shipping logistics executive, Ishmael retired and began to offer his services as a logistics consultant. Ishmael entered into a consulting contract with the general manager at Export Co to provide logistical advice to the company for 12 months. The parties agreed in the contract that Export will pay Ishmael $5,000 per month. Ishmael promised to devote "100 percent of his professional time" to Export during the duration of the contract. Before entering into the contract, Ishmael told Export that he did not think that he needed to provide logistical advice for more than 10 months. He then signed the 12-month contract. What rule would prevent evidence of the conversation from altering the contract if there is a dispute?