 It is a fair question to ask how the physics of the discourse relates to its metaphysics—for example, how the perceptible properties of observable instances of fire (its brightness, lightness and heat, let us say) relate to the form of Fire, an intelligible reality that has no perceptible properties at all. The form of Fire is the non-spatial, non-material, non-perceptible intelligible, eternal and unchanging nature of fire that the Craftsman “imitates,” and in so doing produces spatially extended, material, perceptible, transient and variable instances of fire. Although we are not told what it is about the nature of fire that requires observable instances of it to have just these properties, it is presumably that knowledge that guides the Craftsman to select and assign the four regular solids as he does. Given that the nature of fire is such that any “imitations” of it that appear in the Receptacle must be perceptible as bright, light and hot, the type of solid body that best supports these properties—the tetrahedron, it turns out—should be assigned to fire. And so with the other three kinds (see 55d7–56c7).