The Devil's Proof (悪魔の証明 Akuma no Shōmei) is the logical dilemma that while evidence will prove the existence of something, the lack of evidence fails to disprove it.
The Devil is used to illustrate the principle, but as a legal term it's often applied in IP rights suits involving black box technologies, such as patented chemical processes for production and software source code. In court the dilemma is resolved by either granting plaintiffs rights to gather information, or by subpoena of internal data from the defendant.
In the game between Battler and Beatrice, a declaration in Red Text may be used to resolve the dilemma. Hempel's Raven is used to counter it: if we accept the existence of a Devil enough to accuse it but it remains unavailable for examination, then the only way to prove its guilt is to prove the innocence of everyone else. An inspection conducted with Detective's Authority will also eliminate a Devil's Proof; if evidence isn't presented to prove something, the possibility of it is considered to be eliminated.