If you follow the actions of the three boys in this second chapter of the novel, you will see that this statement is true. Ralph is rational in that he conducts the meeting, establishes rules, and assigns jobs.
Jack is more irrational. Even though he wants "lots of rules," he is more interested in punishing the offenders than in following rules himself. Jack is the first to try to limit the conch, saying it doesn't "count on top of the mountain." And Jack is disrepectful of Piggy when Piggy has the conch is trying to speak.
Piggy represents the super-ego, or conscience, in that he constantly tells the boys what they should and should not do. He reprimands them for acting like kids, not buidling the fire in a responsible manner, and not building shelters. Piggy is also the one who most respects the power that has been bestowed upon the conch, the symbol of democracy and order on the island. He repreatedly shouts
I have the conch! I got a right to speak.