 The second main section begins with the introduction of the receptacle, a “third kind” alongside the familiar paradigmatic forms and the generated images of the forms (49a1–4, 52a8, d2–4). The receptacle has been seen as serving either as material substratum or as some form of space or as combining both roles. Timaeus’ account of the receptacle presents several interpretive difficulties, some of which will be discussed below. In the “pre-cosmic” state (the state “prior to” the intervention of the Craftsman) the receptacle is subject to erratic and disorderly motions, and moves its contents in turn. Its contents are mere “traces” (ichnê, 53b2) of the subsequently articulated four “kinds” (the so-called “elements”): fire, air, water and earth. Each of these four kinds is made up of geometrical bodies—fire of tetrahedra, air of octahedra, water of icosahedra, and earth of cubes. The Craftsman begins by constructing these four regular solids as the primary corpuscles of each of the four kinds. These solids have faces that are made up (ultimately) of two types of right-angled triangles—the half-equilateral and the isosceles—and it is these triangles that are the “simples” of the physics of the dialogue. Because their triangles are similar (half-equilateral), only corpuscles of fire, air and water may be transformed into one another; while corpuscles of earth are made up of isosceles triangles and are thus excluded from such transformations. Each of the four kinds has properties that are determined by the constitution of their respective corpuscles, and these properties in turn determine how the particles act upon and react to one another. These actions and reactions are ongoing and perpetuate a state of non-uniformity which itself is a necessary condition for motion, i.e., the continuation of the interactions. Although each of the four kinds has a tendency to move toward its own region of space, their being squeezed together in a spherical universe without any gaps leads to the inevitable transformations that occur when their various corpuscles cut or crush one another. Thus it is assured that these migrations are never completed in the sense that there would be a complete separation of the four elements into separate regions. The account proceeds to explain the various varieties of each of the four kinds, and the sensible properties that they and their compounds manifest. An account of sensible properties calls for a preliminary account of sensation (including pleasure and pain), and it is with that preliminary account that this section of the discourse concludes.