What does it mean to know something, rather than merely to believe it?
Suppose you believe there is a fire on the far hill because you can see smoke. Suppose you are right. Have you known it, or have you guessed well?
Pramāṇa — tshad ma in Tibetan, usually rendered as valid cognition
— is the branch of the curriculum that takes this question seriously enough to spend years on it. It asks what makes a mind's grasp of something reliable: which kinds of awareness can be trusted, which cannot, and how to tell the difference from the inside.
It is not a formality before the interesting material begins. It is the reason the rest of the curriculum can be argued about at all.
The root text is Dharmakīrti's Commentary on Valid Cognition (Pramāṇavārttika), written in India in the seventh century. Dharmakīrti was working in a philosophical culture where Buddhist positions had to be defended against Hindu and Jain opponents who did not share their premises — which is why the arguments are built to survive hostile examination rather than to be admired.
That inheritance is why the monastery's courtyard sounds the way it does. The text was written for people who expected to be contradicted.
The first three years are given to three subjects that together form the working machinery of debate.
Collected Topics (bsdus grwa) trains definition and the relations between categories — what it means for one thing to be included in another, to exclude it, or to overlap. Mind and Awareness (blo rig) sorts the ways a mind can take an object: direct perception, inference, correct assumption, doubt, and outright error. Signs and Reason (rtags rig) is the study of what makes a reason actually establish its conclusion.
A monk who has finished these three can state a position precisely, see what follows from it, and identify exactly where an opponent's argument fails. He has not yet studied the path to enlightenment. He has learned how to think about it.
Every later subject is examined by debate, and debate runs on the forms Pramāṇa establishes. A monk arguing about emptiness in his twelfth year is using equipment he built in his first.
It is also the subject that most obviously transfers. Whatever one makes of the Buddhist conclusions, the training in stating a claim exactly, distinguishing what is proven from what is assumed, and conceding cleanly when a reason does not hold is not confined to a monastery.
Back to education and intellectual training, or on to the other treatises: Pramāṇa, Prajñāpāramitā, Madhyamaka, Abhidharma, Vinaya.