A porcupine inspires Brian to build a fire and symbolizes vulnerability because Brian needs to defend himself from the wild animals.
Brian finds himself stranded in the wilderness due to a plane crash with no adults and only a hatchet. At first he has no idea what to do. When a porcupine wanders into his camp, at first it just startles him and he reacts with a reflex. The inevitable result is that the porcupine’s quills get him and he is in terrible pain. It is then that he realizes that he is vulnerable.
I can't take it this way, alone with no fire and in the dark, and next time it might be something worse, maybe a bear, and it wouldn't be just quills in the leg, it would be worse. I can't do this, he thought, again and again. (Ch. 8)
To Brian, the porcupine does not just inspire him to build a fire. It symbolizes his vulnerability. He realizes that unlike the porcupine he has no natural defenses. He can’t defend himself. The next time it might not be just a prick from a few quills that gets him. Something much more dangerous could be lurking out there, and he can no longer sit back and wait to be rescued. Brian realizes that he needs to think on his feet. He needs to defend himself, take charge, and be proactive.
In a dream, Brian is inspired once again. He decides to use sparks from the fire to make a hatchet. He uses the sun gleaming off the hatchet and dried grass. What he is really using is his ingenuity. Brian has decided not to wait around for the adults to come and rescue him. He has taken matters into his own hands, and decided he needs to survive on his own.