Summary
Priors are the expectations a brain already holds before new sensory information arrives. The word comes from Bayesian statistics, where an estimate held in advance is updated against fresh evidence. As the philosophers Wanja Wiese and Thomas Metzinger write: “The prior codes the information the agent already has”. Predictive processing, a framework in cognitive science associated with Karl Friston, Andy Clark and Jakob Hohwy, treats perception this way: the brain constantly predicts what is out there and corrects itself by its errors, mostly outside awareness.
A prior in this sense is a statistical quantity inside one perceiving system, not a belief someone could state.
Rationale
The first concept Part 1 lists for what lies under belief. We borrow the pointer, not the maths: whether whole cultures share priors is still an open question in the science.