The project of grounding necessity in essence often goes together with the model of essence that assimilates the constitutive essence of an object to the definition of it. The paper argues that if the grounding project is to succeed, the definitional model must be questioned. Like any object whatever, a concrete individual is necessarily identical to that individual. It is argued that this necessity can have an essential ground only if the primitive identity property of it or its thisness is (...) essential to it, and the thisness is in its constitutive essence while not in the definition of it. A concrete individual is in essence partly definable but not fully explainable, for the definition of it forms a crucial part of its constitutive essence, but the constitutive essence includes something fundamental beyond definition or explanation. The definitional model must give way to a model of essence that accommodates this. (shrink)