What is the difference between initial and supervening impossibility?

Every contract requires a lawful object and lawful consideration (Unit 1's fourth essential): the promise's purpose and its price must not be forbidden by law, nor defeat its provisions, nor be fraudulent, immoral or opposed to public policy. The importance: the legality requirement is the private bargain's subordination to the public order — the courts will not lend their machinery to illegality (ex turpi causa non oritur actio — no action arises from iniquity): the illegal agreement is void, unenforceable by anyone, and the parties are left where the law finds them.