If its humanism, however weak, is the chief merit of Greco-Roman historiography, its chief defect is substantialism. By this I mean that it is constructed on the basis of a metaphysical system whose chief category is the category of substance. Substance does not mean matter or physical substance; indeed many Greek metaphysicians thought that no substance could be material. For Plato, it would seem, substances are immaterial though not mental; they are objective forms.
