How does Edmund try to make Peter doubt the goodness of the faun

Edmund suggests to Peter that the robin might be setting a trap, but Peter says that robins are usually good birds in stories. Edmund then asks how they can be sure that Mr. Tumnus is a good faun, and that the "Queen" is evil? Peter responds that they can be sure that Mr. Tumnus is good because he saved Lucy, but Edmund suggests that the faun may have merely been pretending to save her.