Full context:   In Chapters 6 and 7 of the famous treatise On the Nature of Man (before 400 BCE), attributed by the Greeks to Hippocrytus or to his son-in-law Polybius, although the elements themselves are not mentioned, the elemental qualities are associated with the four seasons and the four humors:       In the Timaeus Plato explains the necessity of supposing the existence of four elements, not only three, to constitute the body of the Universe: "If then there were a surface having no depth that was supposed to become the body of the Universe, a single midpoint would have sufficed to link the extremities and the center; but the Universe is solid in nature as suited it to be; so, with solids, it is never one, but always two midpoints that are necessary to harmonize them."