When Simon is alone in the jungle, he witnesses the creation of the lord of the flies as Jack places the slaughtered sow’s head on a stick, which is then wedged into some rocks.
The sight of the pig’s head disturbs him. He watches as innumerable flies cover the decaying head, and he gazes in the half-torn jaw and black, lifeless eyes. As he does so, Simon begins to hallucinate. He perceives the head communicating with him, taunting him with suggestions like the following: “Run away, said the head silently, go back to the others.”
Simon responds aloud to the pig’s remarks about the beast within all of the boys. At the end of the chapter, Simon loses consciousness after the lord of the flies threatens to kill him.
Based on the clues Golding included in this chapter, it can be inferred that Simon has a condition that causes him to have seizures, which are preceded by hallucinations and other sensory issues. Thus, one could interpret his conversation with the pig’s head as a figment of Simon’s imagination.