"What I mean is... maybe it's only us."'
'However Simon thought of the beast, there rose before his inward sight the picture of a human at once heroic and sick.'
'"I don't believe in the beast."'
S is the first to suggest that the beast is internal.
He somehow guesses that the beast is the parachutist.
He has good foresight and a deeper, wider understanding of events that the other boys (even Piggy) lack.