“Every negation implies an affirmation.”
- Norman Geisler
Geisler is a Christian apologist, so his context was those who argue the New Testament cannot be considered reliable because history can’t be known.
The problem with the argument is the negation (cannot be considered reliable) implies an affirmation (the history of the New Testament is known). However, the arguer has already claimed history can’t be known.
It’s what Geisler calls a “self-defeating argument.”