 Timaeus prepares for the transition to the second main part of his discourse by pointing out that while a causal story of the sort he has been telling so far is indeed concerned with what is properly the cause (aitia) of the universe’s generation, that story is not by itself sufficient and must extend to an account of “contributing causes” (sunaitiai or summetraitiai, 46c7, 46e6, 68e4–5) as well. The discourse must provide an account of the various physical structures that are necessary for and support the achievement of the purposes of Intellect. The properties possessed by these various structures are determined by their constitutions as a matter of “Necessity,” and it is not open to the Craftsman to change or eliminate the properties of these structures. The properties allow (or disallow) certain processes desirable to the Craftsman, and to the extent that Intellect achieves its desiderata, it succeeds in “persuading” Necessity (48a2–5). It is the role of the second major part of the discourse to set forth these contributing causes.