Plato's Cosmology: The Timaeus

The Forms vs. the Cosmos The world of Forms 1. The world of being; everything in this world "always is," "has no becoming," and "does not change"(28a). 2. It is apprehended by the understanding, not by the senses. The physical world (= the Cosmos) 1. The world of becoming; everything in this world "comes to be and passes away, but never really is" (28a). 2. It is grasped by opinion and sense-perception. 3. The cosmos itself came into being, created …

…the corpuscles. If you press him to say what happens to that portion of the matter within the icosahedron which cannot be enclosed within the equivalent surface area of smaller polyhedra, Plato would say that there is no such matter: after creation matter exists only in the form of space encapsulated by polyhedra; what is not thus encapsulated is empty space, which becomes matter when captured by envelopes of the approved stereometric form.