An objection says that the secondary transmitter depends on the original teacher: if the teacher no longer remembers the report, why accept the pupil?
The response distinguishes between positive negation by the source and the source's lack of current knowledge. The latter does not logically refute the pupil's affirmative account. Under appropriate conditions, a trustworthy student's narration can therefore stand.
An analogy with legal testimony is debated as well. A principal witness's explicit denial can undermine testimony derived from him, whereas hadith transmission has specific rules governing preservation and hearing. The analogy must not erase the distinction between forgetting and denial.