 For those who decide to model what is ultimate in the face of or informed by these challenges, there are several common categories, or “model types” as Philip Clayton once called them, which distinguish kinds of models of what is ultimate from each other. Knowing these categories can organize what might be an otherwise haphazard array of models, in something like the way knowing what an oak, maple and birch are can help sort the sights on a walk through a forest. The categories of ultimacy are best grasped by framing them with a question. For example, Hartshorne and Reese (1953) categorized models of God in particular as the logically possible variations on a theme of five questions, whose positive answers get symbolized by ETCKW: