It's part of an argument Piggy makes about being rational and logical on the island. Piggy is always excellent at working things out, and he is basically arguing that you can't work with imaginary evidence.
You have doctors for everything, even the inside of your mind. 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. In a year or two when the war’s over they’ll be travelling to Mars and back. I know there isn’t no beast—not with claws and all that, I mean—but I know there isn’t no fear, either.”
Ralph moved restlessly.
“Unless we get frightened of people.”
There are doctors for everything, Piggy says. Everything can be worked out. We can ever work out how to get to Mars based on what we see and what we know. Life is to do with empirical information, things that we know, and conclusions that we draw.
So, Piggy continues, he knows there isn't a beast with claws. The only possibility is - and, of course, as anyone who's read the book will know, it's the right one - that they are frightened of what IS on the island: themselves, each other, and the darkness of man's heart.
In Chapter 5, Ralph holds an assembly and addresses the issue regarding the identity of the beast. Ralph mentions that he doesn't believe that there is a beast on the island, and Jack agrees with Ralph's assessment. When Piggy takes the conch, he says that he doesn't believe that a beast exists either. Piggy then attempts to solve the issue concerning the beast pragmatically by stating,
"You don't really mean that we got to be frightened all the time of nothing? Life...is scientific, that's what it is" (Golding 118).
Piggy is by far the most intelligent boy on the island and tries his best to rationalize the existence of the beast by thinking "scientifically." He comes to the conclusion that there could not possibly be a beast with claws living on the island. Piggy also realizes that fear is only a feeling. He then attempts to persuade the boys that their fear is irrational unless they become afraid of each other.