I think you mean that Simon is the only one to doubt the existence of the "beast". As the boys walk, Simon thinks about whether the beast does exist. His contemplation of the beast's existence is described. "However Simon thought of the beast, there rose before his inward sight the picture of a human at once heroic and sick." For the other boys, the beast has become an actual form, the corpse on top of the mountain. For Simon, he can't rationalize the beast's existence since they haven't seen it. When Simon imagines the beast, he sees the image of a man who is either heroic or sick. He's unable to put into words what he thinks about the beast, however. Simon and Ralph come close to talking about the beast, but they don't because Simon walks into a tree while deep in thought. The beast is a symbol of the uncivilized actions and savagery of the boys as they lose order in their lives.
sorry about that. thats what i meant to put.