What is Simon saying when he thinks the beast may be inside the boys themselves?

When Simon suggested that, and I quote... "Maybe.... maybe, there is a beast." The assembly 'cried out savagely.' When Ralph asked in shock and amazement "You, Simon? You believe in this?" Simon replied "I don't know..." Then a storm of protests erupted from the boys, shouts included; "Sit down! Shut up!" And "Sod You!" When Simon clarified his statement by saying "What I mean is... maybe it's only us." Piggy was so shocked that he shouted, "Nuts!"