Summary
A belief is the attitude of taking something to be true, whether or not it is. It’s a basic term in philosophy of mind and epistemology; the Stanford Encyclopedia of Philosophy describes it as the attitude we have “whenever we take something to be the case or regard it as true”.
Believing doesn’t require thinking about it. Philosophers separate occurrent beliefs, the few in mind at any moment, from dispositional ones held unattended, like the belief that you have a head. Some accounts go further and define belief by what it disposes us to do: to believe something is to be ready to act as though it were so.
Rationale
Part 1 invites us to “look beneath conscious beliefs and opinions”, so belief is here as the contrast, not another name for deep disposition. Philosophy’s dispositional beliefs blur that line a little.