A more thorough explanation:

Impossibility is a defense used in criminal and contract law. It excuses a person from liability or non-performance when an unforeseen event occurs that makes it impossible to fulfill their obligations.

Under contract law, impossibility can be used as a defense when an unforeseen event occurs after the contract is made that makes performance impossible. For example, if a person agrees to clean a theater for a year but the theater burns down, the person is excused from performing the rest of the contract because the contract was based on the theater's existence.

Under criminal law, impossibility is a defense in some jurisdictions that can remove liability for certain attempted crimes. There are two types of impossibility:

• Factual impossibility: This is when a person is unable to complete a crime due to a non-existent factual circumstance. This does not function as a defense. For example, if a person tries to steal a car but the car is locked and they cannot get in, this is factual impossibility.
• Legal impossibility: This is when a person believes their actions are a crime when they actually are not. This can function as a defense and protect the defendant from attempted liability. For example, if a person tries to buy drugs from an undercover police officer who is not actually selling drugs, this is legal impossibility.

These examples illustrate how impossibility can be used as a defense in both contract and criminal law. In contract law, the unforeseen event (the theater burning down) made it impossible for the person to fulfill their obligation. In criminal law, legal impossibility (the undercover officer not actually selling drugs) means that the person did not actually commit a crime.