 The account Timaeus gives of the generation of the universe is from the outset based on metaphysical and epistemological principles familiar from the dialogues of Plato’s middle period, particularly the Republic. Introducing the subject of his discourse, Timaeus posits a distinction between what always is and never becomes and what becomes and never is (27d5–28a1). He goes on to connect each with its familiar epistemological correlate (28a1–4): the former is grasped by understanding (noêsis) involving a reasoned account (logos), and the latter by opinion (doxa), which involves unreasoning sense perception (aesthêsis alogos). Although Timaeus does not here name the types of entity that satisfy these descriptions, the reader familiar with the Republic will call to mind the distinction between forms and sensibles (518c, 534a). The role of what is as the model after which the Craftsman designs and constructs the universe (29a) recalls the role of the forms as models for the philosopher-rulers to imitate in exercising their statecraft (Rep. 500b–501c). It is not until much later in Timaeus’ discourse (51b7–e6) that forms are mentioned for the first time, and their existence is argued for on the basis of the distinction (itself supported by argument) between understanding and (true) opinion. And the identification of what becomes with sensibles (in this case the universe as an object of sense) is readily made at 28b7–c2 (see 5. in the argument below).