This is a great question. In order to answer this question, you have to know something about Simon, Jack and Ralph.
Ralph and Jack are polar opposites. The former represents order and civilization at its best. And Jack represent chaos. Just a reading of the book will make this point clear as Ralph seeks to get the boys off the island and seek an orderly way of life, whereas Jack wants to engage in the hunt and has little regard for others. For him might makes right.
Simon, on the other hand, is very different. He does not really fit in, as he is the most spiritual of the boys. He certainly is the Christ representation in the book. As such, he is able to deal with fear in a way that the others cannot. So, he can entertain the idea the the beast is within. This represents the Christian understanding of human depravity. This is why Simon famously says: “What I mean is . . . maybe it’s only us.”
EDIT: "acknowledge/mention the BEAST..."