We may now pass from undertakings that certain facts are true at the time of making the contract, to undertakings that certain facts shall be true at some later time,--that is, to promises properly so called. The question is when performance of the promise on one side is a condition to the obligation of the contract on the other. In practice, this question is apt to be treated as identical with another, which, as has been shown earlier, is a distinct point; namely, when performance on one side is a condition of the right to call for performance on the other. It is of course conceivable that a promise should be limited to the case of performance of the things promised on the other side, and yet that a failure of the latter should not warrant a rescission of the contract. Wherever one party has already received a substantial benefit under a contract of a kind which cannot be restored, it is too late to rescind, however important a breach may be committed later by the other side. Yet he may be [334] excused from going farther. Suppose a contract is made for a month's labor, ten dollars to be paid down, not to be recovered except in case of rescission for the laborer's fault, and thirty dollars at the end of the month. If the laborer should wrongfully stop work at the end of a fortnight, I do not suppose that the contract could be rescinded, and that the ten dollars could be recovered as money had and received; /1/ but, on the other hand, the employer would not be bound to pay the thirty dollars, and of course he could sue for damages on the contract. /2/ But, for the most part, a breach of promise which discharges the promisee from further performance on his side will also warrant rescission, so that no great harm is done by the popular confusion of the two questions. Where the promise to perform on one side is limited to the case of performance on the other, the contract is generally conditioned on it also. In what follows, I shall take up the cases which I wish to notice without stopping to consider whether the contract was in a strict sense conditioned on performance of the promise on one side, or whether the true construction was merely that the promise on the other side was limited to that event.