 Further developments come in the form of Gibbard’s arguments for the resulting theory: Gibbard suggests that people need to plan and need ways to think about and represent their plans. He argues that they would also need to think about what to do from the perspectives of various other people and to formulate plans for arbitrary situations they might find themselves in. If these claims are right, a language might naturally develop in order to make such thinking easier. A planning language modeled by the sets of world-plan pairs would serve very well.  And in actual use it would operate much as our actual normative language does. It is therefore reasonable to conclude that our actual normative language is of this sort.  Furthermore conceiving of these attitudes as involving contingency plans for descriptively specifiable circumstances would allow us to explain the supervenience of the moral on the descriptive.  The thoughts represented by the fact-prac world apparatus represent such contingency plans. So the supervenience of the normative on the descriptive falls naturally out of the resulting story (Gibbard 2003).