What does Brian mean that it was a mental thing in the book hatchet?

Brian meant that the mental aspect of survival was just as important, if not more so, than the physical aspect. He had to overcome his fears, doubts, and insecurities in order to survive. He had to learn how to think clearly and rationally, even in the most difficult of circumstances. He had to find ways to keep himself motivated and focused, even when he felt like giving up. In short, Brian had to learn how to be mentally strong in order to survive physically.