Timaeus is a theoretical treatise of Plato, written circa 360 B.C., which conjectures on the composition of the four elements which the ancient Greeks thought made up the universe: earth, water, air, and fire.
Plato conjectured each of these elements to be made up of a certain Platonic solid: the element of earth would be a cube, of air an octahedron, of water an icosahedron, and of fire a tetrahedron. Each of these perfect polyhedra would be in turn composed of triangles. Only certain triangular shapes would be allowed, such as the 30-60-90 and the 45-45-90 triangles. Each element could be broken down into its component triangles, which could then be put back together to form the other elements. Thus, the elements would be interconvertible, so this idea was a precursor to alchemy.
Plato's Timaeus posits the existence of a fifth element (corresponding to the fifth, remaining, Platonic solid) called quintessence, from which space itself is made out of. (Cf. the "bulk" of brane cosmology.)
See also: Johannes Kepler.