Final answer:
Simon suggests that the 'beast' may be inside the boys themselves, representing the dark side of human nature that can destroy mankind.
Step-by-step explanation:
Simon suggests that the 'beast' may be inside the boys themselves, meaning that it represents the dark side of the human personality that has the power to destroy mankind. This idea is similar to Descartes' concept of the evil demon, which can manipulate our experiences and make us believe things that may not be real. By suggesting that the 'beast' is within them, Simon is implying that the boys' own inner darkness and fears are what are ultimately causing their downfall.