Piggy is the much-hated, overweight boy on the island who represents exactly what the boys are turning away from: rational thinking. It is Piggy who determines that they need to count the number of boys on the island. It is Piggy who decides to use the conch to assemble the boys. It is Piggy who wants to learn everyone's name. It is Piggy who tells the group that the beast can't be real. And it is Piggy who notes that the signal fire should be moved to increase their chances of survival.
Piggy's ideas aren't valued because of his weight and physical challenges; the other boys find him pathetic and ridiculous, never to be taken seriously. Because Jack successfully leads the group of boys increasingly toward utter savagery, the boys display a growing sense of irritation and even anger toward rational thinking. Instead, they react emotionally and in a mob mentality.
In Chapter 8, notice how the pigs Jack hunts are described:
The pigs lay, bloated bags of fat , sensuously enjoying the shadows...
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