A quick definition of subjective impossibility:

Impossibility means that something cannot happen or be done. In contracts, impossibility can excuse performance if the subject or means of performance has deteriorated, has been destroyed, or is no longer available, the method of delivery or payment has failed, a law now prevents performance, or death or illness prevents performance. However, increased or unexpected difficulty and expense do not usually qualify as an impossibility and thus do not excuse performance. There are different types of impossibility, such as factual impossibility (when the illegal act cannot physically be accomplished) and legal impossibility (when what the defendant intended to do is not illegal even though the defendant might have believed that he or she was committing a crime). Supervening impossibility arises after the formation of a contract but before the time when the promisor's performance is due, and arising because of facts that the promisor had no reason to anticipate and did not contribute to the occurrence of.