   On the basis of the fact that, unlike (9), (10) can serve to express two distinct counterfactual beliefs, Kripke (1972) hypothesizes that a proper name is what he calls a rigid designator. ‘Jacques Chirac’ in (9) is rigid for in all possible worlds it refers to one and the same concrete individual, i.e., the individual to whom it in fact refers in the actual world. By contrast, the definite description ‘the president of France elected in 2002’, which happens to express a contingent property of Chirac, is not a rigid designator because it does not pick one and the same individual in all possible worlds. When evaluated with respect to different worlds, it is true of different individuals. Of course, some definite descriptions (e.g., ‘the square root of 21’) do express essential properties of what they are true of (in this case, a number). According to Kripke (1980), unlike proper names, whose rigidity is de jure, definite descriptions expressing essential properties of an object are rigid de facto. For further discussion, see the entry on  reference.