Abstract

Ch. 9 initiates an investigation of the similarities between proper names and natural kind terms by arguing that although most proper names are clearly rigid, application of the notion of rigid designation to natural kind terms is highly problematic. Because natural kind terms come in a variety of syntactic and semantic types, and often function as predicates rather than singular terms, it is doubtful than any interesting notion of rigidity applies to them all. It is argued in particular that there is no notion of rigid designation for predicates that (1) is a natural extension of the notion of rigidity for singular terms, (2) is such that simple natural kind predicates are standardly rigid whereas many other predicates are not, and (3) plays the role imagined by Kripke in explaining the necessary _a posteriori_ status of theoretical identities like _Water is H 2 O_ and _An object x is hotter than an object y iff x has a higher mean molecular kinetic energy than y_. Because of this, a new explanation of the status of these sentences is needed.