The most influential judicial voices on the parol evidence rule are Roger Traynor and Richard Posner. Traynor pieced together aspects of positions championed by the antipodal titans of contracts, Arthur Corbin and Samuel Williston. Posner cuts through tangled doctrinal webs to show how the unifying talisman of the doctrine is credibility. Everything in parol evidence rule doctrine, in this formulation, can be understood in terms of two categories of evidence: subjective and objective. While the Traynor composite blended aspects of the titans of contracts into an incoherent stew, the Posner composite unites the central theme of the titans' positions, holding some promise of at last bringing clarity to a seemingly intractable body of contract law.
Costs associated with various formulations of the parol evidence rule and tradeoffs they entail include costs of perfect drafting compared to costs of judicial error from imperfect drafting. While such a comparative cost approach to thinking about the parol evidence rule may be sound in theory, it is impractical and paradoxical in operation. It requires a judge to evaluate simultaneously costs of perfect drafting and the risk that she or he will commit an error in applying the parol evidence rule. A credibility-centered parol evidence rule can enhance judicial understanding of both sides of this calculus while sidestepping the paradox.
Lawrence A. Cunningham, Toward a Prudential and Credibility-Centered Parol Evidence Rule, 68 U. Cin. L. Rev. 269 (2000).