Many philosophers have claimed possible worlds semantics is incoherent because of insoluble problems involved in the notion of identifying a single individual in different worlds. One frequent approach to trans-world identification has been to assume that all the possible worlds, complete with their populations, are described by means of qualities alone prior to our considering the question of identification of the same individual in each world in which it exists. If we interpret possible worlds semantics in this way, trans-world identification could only be accomplished on the basis of some properties the individual has uniquely in every world in which it exists. This becomes problematic since the individual doesn't have the same properties in every world. In ‘Naming and Necessity’ and ‘Identity and Necessity’ Saul Kripke rejects such an account of both possible worlds and trans-world identification, developing an alternative interpretation of the new semantics. His approach involves a distinction between referring expressions which designate different individuals in different worlds according to the distribution of properties within each world and referring expressions which designate the same individual in every world.