Can the Impossible be proven?
We have to talk here about the "Probatio Diabólica" or "Diabolical Probatio" an expression derived from Latin with which the Spanish Courts have created a figure that aims to guarantee and protect one of the fundamental pillars of the procedural rules of Spanish Law: the burden of proof. This so called in Spanish Law: burden of proof/evidence ("carga de la prueba") is regulated in articles 216 and 217 of our Civil Procedure Law.
The diabolical probatio protects the procedural party that alleges a right or a situation from which a legal consequence that is impossible to prove is derived, since it is in the hands of the other procedural party to prove such an extreme.
For example, the party that performs a contract that would claim that the other party has not paid the agreed price and that the non-performing party will have the burden of proving that it has paid the price, the plaintiff would never be able to prove that the defaulter has not paid the price, since it is impossible to prove a negative fact.