A farmer sees the water level of a river rising rapidly and from that concludes that it must have rained heavily upon the hills. Which of the instruments of knowledge (pramana) is used in this example?

The farmer reaches an unseen fact through a perceived sign, which is inference.

He directly sees only the river rising rapidly.

From the invariable link between heavy rain upstream and a swelling river he concludes it rained on the hills.

Moving from an observed mark to an unobserved cause through their constant relation is inferential knowledge.

This use of a sign governed by invariable concomitance is the pramana of inference.