Final answer:
The most important rule of survival that Brian learns is to stay calm and not panic, which helps him make better decisions in the wilderness.
Step-by-step explanation:
After being struck by the porcupine in the wilderness, Brian, the protagonist of the novel, comes to realize that the most important rule of survival is to stay calm and not give in to panic. This revelation is critical to his survival in the wild, as it allows him to think clearly and make rational decisions in the face of peril.
Brian understands that allowing himself to become overwhelmed by fear can lead to hasty and potentially fatal mistakes. By adopting a calm and composed attitude, he enhances his chances of safely navigating the challenges that come with surviving in the wilderness.