 Once the conclusion that the universe is teleologically structured is settled, the explanatory methodology of the discourse proceeds accordingly. The question that frames the inquiry is not the question: What best explains this or that observed feature of the world? It is rather the question: Given that the world as a whole is the best possible one within the constraints of becoming and of Necessity, what sorts of features should we expect the world to have? This question invites a priori answers, and Timaeus’ arguments about the most general features of the universe as a whole (for example, why it exists, why it is alive and intelligent, why it is unique, why it is shaped and composed as it is) are derived wholly a priori. The answers to these questions are not open to empirical confirmation. But clearly the inquiry is also constrained by features of the universe that are actually observed, and this gives rise, secondly, to questions about the good purposes that are being served by these features (for example, the motions of the heavenly bodies, the psychophysical constitutions of human beings, etc.), and how the features in question accomplish those purposes. For the most part there is a happy coincidence between the features that are required (in answer to the first question), and the features that are actually observed (in answer to the second), and it is part of the genius of the discourse that these are so well woven together. Occasionally, however, the methodology leads to conclusions apparently at odds with observation—for example, the exemption of earth from inter-elemental transformation (see further under “Physics”  below).[14]