 One important aspect of Porta’s physiognomonics is that it is not merely descriptive: bodies may be mapped and their future predicted, but they are nevertheless in a constant state of flux, transition, or mutability. Their essences are not exclusively determined by an invisible substantial form (soul) derived from unchangeable forms—and, in the case of human beings, immortal—as in previous traditions, but (more than ever) result from external circumstances. These environmental influences are decisive for the fate of individuals whose expressions and appearances are unstably shaped. A characteristic quotation illustrates Porta’s understanding of physiognomonics: