Brian; Nature; Everything Else
Okay, we know this may sound like a cop-out, but really just about everything is a guide for Brian. Because he's in such a desperate situation and he's so woefully unprepared (talk about being tested on something you haven't studied for), he clutches at anything at all that can help him to survive.
Think about it: where does Brian get the ideas and skills that allow him to find food, construct a shelter, and craft the tools he needs? Well, first of all, of course, from direct observation of things around him—for instance, he gets the idea to try to catch fish from seeing a kingfisher diving into the lake and coming up with a beakful of dinner. But he also relies on his own memory and ingenuity to guide him toward possible solutions. How many times do we see Brian thinking about something he saw on a television show or read in a book and analyzing how it might apply to his situation? And how often does he remember some tiny tidbit he heard in school only to have it send him off in an unexpected direction? (Shmoop says: Listen to your teachers because you just never know when it will come in handy. Cheesy but true.)
Because Brian is totally alone, he's forced to rely on scraps and fragments from his past as he fights to survive. In his dream it's his father and his friend Terry who try to help him see how he can build a fire, but of course it's Brian's mind itself which is really trying to nudge him toward the solution. If, as they say, knowledge is power, then Brian uses every little bit at his disposal to keep himself alive.