Simon appears to be the wisest amongst the boys, but his deeply sensitive/spiritual nature sees him treated like an outcast. His failure to realise his potential in giving sound advice to the group is sadly demonstrated at the gathering which debated the existence of a beast on the island.
Simon felt compelled to speak, but failed to convey his point.
"Maybe," he said hesitantly, "maybe there is a beast."
"What I mean is.....maybe it's only us."
"We could be sort of...."
Simon became inarticulate in his effort to express mankind's essential illness. (p. 111)
He reaches for the truth, that the beast is not in a physical form roaming the island but is actually inside all of them - a kind of primitive savagery and evil which threatens to emerge given the fertile conditions of isolation and absence of adult leadership. His indecision is repeated later in front of Ralph as the boys set off in a determined effort to act on Samn' Eric's testimony.
Simon mumbled confusedly: "I don't believe in the beast." (p.130)
He fails to elaborate on this and Ralph clearly doesn't take him seriously anyway. Simon's failure to articulate his fears and perhaps help change the course of terrible destruction the boys embark on is one of many tragic twists that occurs in the course of the novel. His final uncovering of the truth leads to the awful fireside climax, where his death finishes all hope of sanity and civility being restored to the boys.