If the liability of the promisor is created by an express promise, subsequent impossibility does not, as a general rule, excuse performance.1 Thus where an insured became insane, and by reason of that fact did not pay the assessments upon his policy, and the policy thereby lapsed, such insanity did not excuse the failure to pay the assessments.2 Certain classes of subsequent impossibility, however, discharge a promisor from liability which is created by his express promise. What classes of such impossibility amount to such discharge, according to the view entertained by modern authorities, depends upon the intention of the parties as deduced from the terms of the contract. If the event which has happened is one which is not fairly within the meaning of the contract, and cannot be assumed to have been contemplated by the parties, such event operates as a discharge.3 This form of stating the rule conforms more closely to the facts than the alternative form of stating it, namely, that the event creating impossibility must be considered to have been intended by the parties as an implied condition subsequent upon the happening of which the contract should be discharged.4 This last method of stating the rule, is inexact, since, in the cases under consideration the event which operates a discharge is not contemplated by either party. Whether an exact classification of the events causing subsequent impossibility, which are not within the fair meaning of the contract as made by the parties, is possible, is a question upon which the courts are not in perfect accord. Some authorities have restricted such events to three classes: (1) Where the impossibility is created by law. (2) Where the continued existence of something essential to the performance is an implied condition of the contract. (3) Where contracts are made for personal services which cannot be performed by the assignee or a personal representative.5 As will be seen later,6 some jurisdictions recognize classes of impossibility as amounting to a discharge which cannot be brought within any one of these three classes, except by a most artificial use of the terms employed. Where the doctrine of subsequent impossibility is recognized, such impossibility acts as a discharge of the contract, but is by no means the same thing as a performance of it. The promisor may invoke the subsequent impossibility as a defense in an action against him for breach of the contract, but he cannot enforce the contract against the promisee as if he had performed it.7 Recovery for work done under a contract may be had where subsequent impossibility discharges the parties from further performance.8 On the other hand, the promisee cannot insist on a different method of performance from that provided by the contract. Under a contract to construct a tunnel, which proves impossible of performance on account of the nature of the ground, the eon-tractors cannot be required to construct a tunnel upon different plans and specifications.9

7 Lang v. Ry., 154 Pa. St. 342; 35 Am. St. Pep. 846; 20 L. R. A. 360; 26 Atl. 370.

1 School Dist. v. Dauehy, 25 Conn. 530; 68 Am. Dec. 371; Anderson v. May, 50 Minn. 280; 36 Am. St. Pep. 642; 17 L. R. A. 555; 52 N. W. 530; Cowley v. Davidson, 13 Minn. 92.

2 Pitts v. Ins. Co., 66 Conn. 376; 50 Am. St. Rep. 96; 34 Atl. 95.

3 Stewart v. Stone, 127 N. Y. 500; 14 L. R. A. 215; 28 N. E. 595,-Dexter v. Norton, 47 N. Y. 62; 7 Am. Rep. 415; Parker v. Maeomber, 17 R. I. 674; 16 L. R. A. 858; 24 Atl. 464.

4Dolan v. Roderers, 149 N. Y„ 489; 44 N. E. 167.

5 Middlesex Water Co. v. Whiting Co., 64 N. J. L. 240; 81 Am. St. Rep. 467; 49 L. R. A. 572; 45 Atl. 692.

6See Sec. 1368. 1369.

7 Remy v. Olds. 88 Cal. 537; 21 L. R. A. 645; 26 Pac. 355.

8Dolan v. Rodgers, 149 N. Y. 489; 44 N. E. 167; Parker v. Ma-comber, 17 R. I. 674; 16 L. R. A. 858; 24 Atl. 464.

9 Milwaukee v. Shailer, 84 Fed. 106; 28 C. C. A. 286.