What occurs that makes Phillip wonder about Timothy's mental state?

In Chapter 11, Phillip starts to wonder whether Timothy is perhaps going a bit crazy, out of his mind from the stress, when the old man suddenly claims that their pet and companion, Stew Cat, might be the cause of their bad luck. Timothy seems to sneak around with a knife, causing Phillip to be horribly worried. The boy tries to make sure Timothy doesn't harm Stew Cat, and it's a tense chapter as...

In Chapter 11, Phillip starts to wonder whether Timothy is perhaps going a bit crazy, out of his mind from the stress, when the old man suddenly claims that their pet and companion, Stew Cat, might be the cause of their bad luck. Timothy seems to sneak around with a knife, causing Phillip to be horribly worried. The boy tries to make sure Timothy doesn't harm Stew Cat, and it's a tense chapter as the animal remains missing.

Finally, as the chapter comes to a close, Phillip realizes that Timothy has been carving an image of a cat out of wood and has placed nails in that image, trying to symbolically kill the bad luck that has plagued them. Even odder, Timothy has temporarily placed Stew Cat on their raft in the water while he did this task, so that the cat would be physically removed from the island while Timothy's work dispelled the "jumbi" (bad luck) caused by the feline.

"Maybe all that had happened was beginning to work on the old man's mind. Maybe I was stranded on a tiny, forgotten island in the Caribbean with a madman. If he harmed Stew Cat because of some silly jumbi thing, I knew he might also harm me," Phillip muses. 

This incident is important to the novel because it shows one of Timothy's weaknesses; in this episode, Timothy genuinely gives Phillip cause for worry and additional stress. The chapter, then, makes Timothy's character more realistic and believable. Throughout most of the novel, Timothy is nearly angelic, showing constant patience to Phillip and providing for him in knowledgeable, caring ways. This flaw of Timothy's (the belief in the silly concept of "jumbi" and that it can be controlled) helps round out his character and make him more human.