Epistemology for beginners
A lot of it is quite unconscious. When he talks, for example, he has no idea how he does it or where it's going and yet it mostly comes out making sense. Brian comes across as a complicated normal person, sometimes even an expert. Sometimes he finds the things he says surprising and brilliant. The bright little beam of his consciousness may or may not be paying attention, but the machinery in his head is busy anyway.
But not all. Occasionally, Brian bumps into something that niggles away and makes him wonder. Like, do slugs hear tiny violins when they get all romantic? To many people this is exactly the sort of useless junk that gets instantly rejected. But for Brian it's a profound question; if slugs get all poetic about each other, then our experience is really not that different to their's.
Epistemology is the study of knowledge - What is it? How does it work? To what extent can it be true? How much do we have? What are the limits? Stuff like that.
There are lots of different ideas about it, but in the end they're all based on how we see ourselves.