Probatio diabolica

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Probatio diabolica (Latin: "devil's proof") is a legal requirement to achieve an impossible proof. Where a legal system would appear to require an impossible proof, the remedies are reversing the burden of proof, or giving additional rights to the individual facing the probatio diabolica.

For example, one party might patent a process for manufacturing an item. Another party might then make the item. The patent holder would normally have to show that the patented process had been improperly used; this is a probatio diabolica since on the face of it the patent holder has no information on what process was actually used, and this could render the patent useless. Two possible solutions exist:

• the burden of proof is reversed by presuming that the second manufacturer has improperly used the patented process, unless or until he demonstrates that he has used some other process; or
• the patent holder is given discovery rights, enabling him to get information from the second manufacturer on the process actually used.