A more thorough explanation:

Definition: In rerum natura (in reer-uhm nuh-tyuur-uh), is a Latin phrase used in law to mean "in the nature of things" or "in existence". It was often used in a dilatory plea alleging that the plaintiff was a fictitious person and therefore not capable of bringing the action.

• John's lawyer argued that the lawsuit should be dismissed because the plaintiff did not exist in rerum natura.
• The court rejected the defendant's argument that the contract was void because the subject matter did not exist in rerum natura.

These examples illustrate how the phrase in rerum natura is used in law to refer to something that exists in reality, rather than being a fictional or imaginary concept. It is often used to challenge the validity of a claim or contract by questioning whether the subject matter actually exists.