This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
Operative impossibility may be classified according to the time at which the impossibility exists, and according to the nature of the means by which performance is rendered impossible. With reference to the time at which the impossibility exists, impossibility may be classed as impossibility which exists when the contract is made, which is sometimes referred to as original impossibility;1 and impossibility which arises after a contract is made and which operates as a discharge thereof, which is sometimes referred to as subsequent impossibility or supervening impossibility.2 The importance of the distinction between the two classes grows out of the fact that original impossibility prevents the existence of the contract,3 while subsequent or supervening impossibility does not prevent the existence of a contract, but operates as a discharge of a contract which was originally valid and enforceable.4 For this reason in some jurisdictions at least, quasi-contractual rights arise when a contract which was originally valid and which has been performed in part has been discharged in this way.5
3 See Sec. 2696 and 2703 et seq. 4 See Sec. 2696 and 2703 et seq.
With reference to the means by which a contract is rendered impossible, impossibility may be classed as impossibility caused by certain facts,6 and impossibility caused by the act or operation of the law or of the state.7 It may be noted in this connection that while subsequent or supervening impossibility may be due either to the existence of certain classes of facts,8 or to the operation of the law or the act of the state,9 original impossibility is ordinarily regarded, with few exceptions,10 as due to the existence or non-existence of certain facts.11 Except for a few examples of original impossibility which are explained as being due to the operation of the rule of law,12 a contract which is inoperative when it is made, because it violates certain rules of law, is ordinarily treated as a void or illegal contract,13 and not as a contract which is inoperative because of impossibility.