Wiktionary
negative proof
n. A fallacious judgment that because a premise cannot be proven true, that premise must be false.
Usage examples of "negative proof".
The abuse, complained of to the Othos, was, that a person who was charged with having a false title to an estate, defended himself by a negative proof, declaring upon the Gospels it was not false.
And as no offer hath yet been made to refute the doctrine contained in the former editions of this pamphiet, it is a negative proof, that either the doctrine cannot be refuted, or, that the party in favour of it are too numerous to be opposed.