The cases in which quasi contractual obligation may be claimed to result from the impossibility of performing a contract obviously fall into two classes: - first, those in which the plaintiff is prevented from completing the performance of a condition precedent to the defendant's contractual liability and therefore cannot recover in a contractual action; and second, those in which the defendant is prevented from performing, but because of an implied or constructive condition excusing his default, cannot be held for a breach. The two classes of cases will be separately considered.

Sec. 111. (I) Plaintiff's Performance Prevented: In General

While, as has been explained, the cases falling within the scope of this chapter are those in which the default does not constitute a breach, nevertheless, a failure to perform a condition effectually bars a recovery on the contract. If, however, the plaintiff in misreliance upon the contract has by part performance conferred a benefit upon the defendant, he is entitled to restitution.

Sec. 112. (1) Misreliance On Contract: Assumption Of Risk

The English courts, in the cases now under consideration, as in the cases where one who has committed a breach of contract seeks to recover the value of part performance (post, Sec. 164), refuse to recognize a quasi contractual obligation.1 The precise reason for this attitude is nowhere satisfactorily expressed; but it seems to be that if a quasi contractual obligation were recognized, the plaintiff would be allowed to recover compensation for his part performance in the face of his own agreement that for part performance he should receive nothing.2 In other words, the plaintiff is thought to have voluntarily assumed the risk of failure, for any reason, to perform in full. And a voluntary assumption of risk, as heretofore explained {ante, Sec. 16), is incompatible with misreliance.

1 The phrase "impossibility of performance," as commonly used, includes some cases in which performance is not physically impossible, but is excused because of unanticipated danger to the contractor, governmental prohibition, or other extraordinary after-event which would make the enforcement of the contract unfair. See post, Sec. Sec. 122, 124.

Where the contract contains an express stipulation that in case of the failure of the plaintiff to perform it in full he shall be entitled to no compensation whatever, the view that he assumed the risk appears, with a limitation hereafter to be stated (post Sec. 114), to be sound. But to presume, in the absence of such an express stipulation, that the plaintiff assumed the risk of an event so extraordinary that it excuses him from liability for failure to perform his engagement, seems illogical and unjust. The fair presumption, in such a case, it is submitted, is that the risk of a default resulting from impossibility was not assumed and that the plaintiff relied upon a contract right which, because of his excusable default, became unavailable.