during the argument about the existence of a beast, what conclusion do both sumon and piggy come to?

Simon and Piggy come to different conclusions. Simon echoes one of the themes of the novel. Simon musters up the courage to say that perhaps "the Beast only us". Simon realizes that the most frightening thing on the island is themselves. The evil on the island exists in their own hearts. Piggy thinks a beast can't exist because things "simply wouldn't work". Piggy equates "beasts" to ghosts. The world is logical, according to Piggy, and a "Beast" is not logical. Things like trains and planes would not work if we believe in ghosts/beasts.