What does Piggy mean when he says that life is scientific?
In Lord of the Flies, Piggy represents reason and order and science. So in chapter five, when he says that life is scientific, he is trying to help the boys see reason when they talk about the beast. Ralph has called a meeting to try and get things back in order and the opposite has happened. The boys are talking about the beast and Jack has seen their fear and seen that he's also able to take advantage of that fear to further undermine Ralph's control.
But Piggy is trying to explain, in a scientific way, that there can't be a "beastie." As he points out, there isn't a way for an animal that big to live on the island because there's nothing big enough for it to eat there. As at the beginning of the story when he explains the fact that no one will know where they are because they went off course, etc., his explanations are exactly correct but do not always achieve the intended effect.