As a group of boys searches for the beast in chapter 6, Simon is described as "incredulous," meaning that he does not believe that the beast exists. He finds it difficult to believe that there could be a beast "that [sits] on a mountain-top, that [leaves] no tracks and yet [is] not fast enough to catch" two of the other boys. Simon quite rightly reasons that a creature too slow to catch Samneric could not also be a creature so quick and so light of foot as to leave no tracks.
The island upon which the boys are trapped serves as a microcosm to represent human nature, and Golding's predominant opinion about human nature, as expressed through this story, seems to be that human nature is more animalistic than civilized, more irrational than rational, and more susceptible to fear than to logic. Simon's portrayal as the lone voice of reason helps Golding to convey this view of human nature. His lone voice of reason is drowned out by the fears of the other boys, just as, in Golding's view, reason is more often than not overwhelmed by fear in the average human.
Despite being incredulous as to the existence of the beast, Simon is not confident enough to voice his doubts. He is subdued by what Golding calls "the pressure of personality." This too alludes to Golding's view of human nature, and specifically to the idea that external societal pressures act to suppress individual, dissenting voices. It is too easy for humans to get carried away, as it were, with the tide of opinion and to ignore any voices, like Simon's, that seem to push against that tide.