In Chapter Six of Lord of the Flies, Golding writes,
Simon, walking in front of Ralph, felt a flicker of incredulity--a beast with claws that scratched, that sat on a mountain-top, that left no tracks and yet was not fast enought to catch Samneric. However Simon thought of the beast, there rose before his inward sight the picture of a human at once heroic and sick.
After Sam and Eric wake and discover that the fire which they were to maintain has gone out, they relight it with the smoldering coals, but in the glow of this fire they discover the parachuist who has died. Terrified, they run back to camp, exclaimining that they have seen the beast and it has chased them, clawing at Eric's face. While Simon knows that the dead man is not the beast, but a metaphor for what the boys fear, he cannot figure out how to communicate his intuitive knowledge to the others. He and Ralph--"Something flittered there in front of his [Ralph] mind like a bat's wing, obscuring his idea"--nearly reach a communion of thoughts, but circumstances interfere, and each returns to his own thoughts.
Intuitively, Simon knows that the "beast" is the savagery that is emerging in Jack and the others; he realizes it is humanity that the boys fear, but he cannot articulate his thoughts enough for others to comprehend and accept.