Piggy symbolizes rationalism. He has complete faith in science to solve all problems and eradicate all fears. As he says:
"You don’t really mean that we got to be frightened all the time of nothing? Life,” said Piggy expansively, “is scientific, that’s what it is."
Piggy believes there is a rational explanation for everything. He believes that once we discover that explanation, we will have no reason for fear—and that the explanation can and will be found. His faith in science is so profound that he envisions humankind getting to Mars a few years after World War II ends.
Piggy's rationalism leads him to value law and order highly, because a rational universe is a well-ordered, mechanistic universe. He believes, therefore, in the order represented by the conch shell. To him the conch shell has literal as well as symbolic meaning: if he is holding it, it means he has the right to speak. He can't understand that only social consensus gives the conch that power. He also simply cannot comprehend the primitive, atavistic, irrational impulses Jack unleashes in the boys. The idea that it could be fun or satisfying to don war paint, dance, and kill things simply does not compute for him.
In many ways, Piggy could symbolize the computer: like a computer, he lacks personal magnetism or the power to lead on his own. He can only advise. Like a computer, he thinks rationally and doesn't understand the appeal of primitive emotions.