In chapter five, titled "Beast from Water," Ralph admits to himself that Piggy is smarter than he is. Ralph is about to call an assembly together, the first truly serious one, and he's trying to work out how to proceed appropriately. After spending some time thinking, while walking along the beach and noticing the changes that have occurred since they arrived on the island, Ralph begins to plot out how the meeting should go. It's in this section where Ralph first admits that Piggy is gifted with a type of thinking that doesn't come naturally to Ralph.
"Only, decided Ralph as he faced the chief’s seat, I can’t think. Not like Piggy." (110)
Ralph proceeds to acknowledge that Piggy can think, despite his girth. This adds value to Piggy, particularly since that skill, that of thought, is valuable in a chief. Ralph is struggling in this scene to come to the right decision and he worries that he might make the wrong one. As he continues to ponder the decision making process, wisdom, and how to best survive on this island, Ralph recognizes that Piggy doesn't seem to struggle to think in the way that Ralph does. This is an acknowledgement to Piggy's superior intelligence.