Which of the following Indian thought systems holds that the universal relation in inference (anumana) cannot be determined because there is always room for doubt about unobserved cases?

The materialist Lokayata (Carvaka) school denies that a universal relation for inference can be fixed.

Inference rests on vyapti, an invariable tie between the mark and what is inferred.

Such a tie would have to hold for all cases, including those never observed.

The Carvakas argue that unobserved cases always leave room for doubt.

Since the universal cannot be guaranteed, they refuse inference as a valid means of knowledge.

This scepticism about vyapti is the hallmark of the Lokayata position.