"'What I mean is...maybe it's only us.' [...]Simon became inarticulate in his effort to express mankind's essential illness" (89).
In Chapter Five, 'Beast from the Water,' the boys brainstorm ideas about what the beast could really be. Suggestions from the boys range from sea monsters to ghosts, but Simon suggests that "maybe there is a beast" (89). Of all the boys, he seems to understand best the true nature of the beast on the island that the boys really need to worry about--themselves.
Even though Simon is unable to explain himself clearly at the assembly, Golding uses the conversation between the boys to develop the theme of man's inherently evil nature. The "essential illness" is mankind's inability to refrain from deceit and evil. Humanity is naturally susceptible to corruption.