This is a great part of the book because it has a lot of subtleties to it. Ralph, Jack, Piggy, and Simon do not really believe in a "beast." It is mostly the "littluns" who conceptualize this creature:
“But that’s littluns’ talk. We’ll get that straight. So the last part, the bit we can all talk about, is kind of deciding on the fear.”
“We’ve got to talk about this fear and decide there’s nothing in it. I’m frightened myself, sometimes; only that’s nonsense! Like bogies."
"There aren’t any beasts to be afraid of on this island.”--Jack
“I don’t agree with all Jack said, but with some. ’Course there isn’t a beast in the forest. How could there be? What would a beast eat?” --Ralph
"Of course there isn’t nothing to be afraid of in the forest. Why—I been there myself! You’ll be talking about ghosts and such things next."--Piggy
“Maybe,” he said hesitantly, “maybe there is a beast. What I mean is. . . maybe it’s only us.””--Simon
The litluns, though, are unconvinced. They believe that a best is a possibility because it could come out of the sea and return to it at will. Thus, it would not have to have a home on the island and could have all the fish it wanted to eat.
Jack, though, is clever enough to use the litlun's fear as a way to increase his power. He intellectually knows there is no beast, but he uses the idea of the beast as a way to create fear in the camp. Why? Because he and his hunters can offer protection from the fear and this strengthens his hand.
“Bollocks to the rules! We’re strong—we hunt! If there’s a beast, we’ll hunt it down! We’ll close in and beat and beat and beat—!”--Jack
So, that's about it. Ralph and Piggy don't believe in the beast and won't tolerate speculation otherwise. Jack doesn't believe in the beast either, but leaves an opening he can exploit for his own power. Simon doesn't believe in a physical beast, but he is open to the idea that the beast is something inside all of them, some darker nature. It is all a difference in perception, Machiavellian scheming, and practical thinking. It just goes to show that many different people can look at the same thing and come away with different conclusions.