We derive the rate-form spatial equilibrium system for a nonlinear Cauchy elastic formulation in isotropic finite-strain elasticity. For a given explicit Cauchy stress–strain constitutive equation, we determine those properties that pertain to the appearing fourth-order stiffness tensor. Notably, we show that this stiffness tensor \({\mathbb {H}}^{{{\,\textrm{ZJ}\,}}}(\sigma )\) acting on the Zaremba–Jaumann stress rate is uniformly positive definite. We suggest a mathematical treatment of the ensuing spatial PDE system which may ultimately lead to a local existence result, to be presented in part II of this work. As a preparatory step, we show existence and uniqueness of a subproblem based on Korn’s first inequality and the positive definiteness of this stiffness tensor. The procedure is not confined to Cauchy elasticity; however, in the Cauchy elastic case, most theoretical statements can be made explicit. Our development suggests that looking at the rate-form equations of given Cauchy-elastic models may provide additional insight to the modeling of nonlinear isotropic elasticity. This especially concerns constitutive requirements emanating from the rate formulation, here being reflected by the positive definiteness of \({\mathbb {H}}^{{{\,\textrm{ZJ}\,}}}(\sigma )\) .
