 Kripke semantics gives predicates different extensions in different worlds. Thus, for example, the extension of the predicate ‘is a philosopher’ was empty in all worlds (times) before the dawn of civilization and more recently has varied. For rigidly designating terms, this raises the question of how to evaluate atomic formulas at worlds in which their referents do not exist. Is the predicate ‘is a philosopher’ satisfied, for example, by Russell in worlds (times) in which he does not exist—times such as the present? The general answers given to such questions determine whether a Kripke semantics is positive, negative or neutral.