pramāṇānāṃ pramāṇatvaṃ yena cānyena sādhyate / tasyāpyanyena sādhyatvād anavasthā prasajyate //
If the validity of a means of valid cognition is to be proved by another [means of valid cognition], and if that [other one's validity] too needs to be proved by yet another, then an infinite regress would result.