Simon is the only one of the boys who sees the 'beast' for what it truly is.
In chapter five, it is Simon who has the insight of "maybe it's only us." He clarifies to the group that perhaps the beast is only the boys themselves. This early insight that perhaps the beast is merely an imaginative construction reveals the depth of Simon's ability to think critically.
Chapter Six confirms the reality of the beast. Sam and Eric have seen it for themselves as they tended the fire on the mountain. When Ralph calls a council meeting the following morning, Sam and Eric tell of a horrible monster. Again, it is Simon who sees the situation with enough logic and insight to know that the boys' account does not describe a real monster. If it were truly as horrible as Sam and Eric claim, then it probably would have devoured the boys.
When Simon tries to visualize the beast "there arose before his inward sight the picture of a human at once heroic and sick" (Golding 93). Simon's perception is clear enough to realize this, because out of the group, Simon is the only one thinking logically, instead of emotionally.