The book also emphasizes prototype theory, which is says humans often define things in terms of a "best example" of a category rather than with necessary and sufficient conditions. For instance, Americans understand "bird" by thinking of an animal about the size of a robin that flies and has wings and feathers. Although penguins are birds, they are not the best examples or prototypes. Sanders applies that approach to definitions of God and Christianity and explains why Jesus is so central to Christian moral and spiritual formation.