 A deeper problem is that Scanlon’s defense of a regime that enforces contractual expectations does not fully vindicate the law, since it presupposes moral constraints that have no legal analogue. Scanlon (2001: 105) suggests that enforcing a promisee’s expectation is appropriate only if the compensatory burden is not “excessive” and only if the promisor had “adequate opportunity” to avoid it based on an “adequate understanding” of their situation at the time of promising. But the law does not qualify the promisor’s duty in these ways. A promisor might have every intention to comply with a promise and yet later find that, due to events she reasonably did not foresee, the cost of her performance has risen dramatically or the value to her of the other side’s promise has fallen. It is hardly obvious that promise-breaking in such cases violates the duty not to harm (could the promisee reasonably expect the promisor to keep her promise in this scenario?) or that it would be fair to make the promise-breaker liable for the promisee’s full expectation (did the promisor have a reasonable opportunity ex ante to avoid the burden?). While the law excuses breach if the unexpected costs of performance are extraordinary (under the doctrines of “impossibility” and “impracticability”, see R2: §261) or if the decrease in the value of the bargain is similarly substantial (see “frustration of purpose”, R2: §269), these excuses are only available in exceptional circumstances (a leading case on impracticability, Alcoa v. Essex (1980), involved unexpected costs greater than $50 million). Historically, the common law’s pacta sunt servanda (“agreements must be kept”) principle used to be enforced quite unforgivingly (see Paradine v. Jane (1647)); and while courts now recognize a broader category of excuses for non-performance, the law’s remedial demands are not so clearly consistent with Scanlon’s moral framework.