Non-rigid designator
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In the philosophy of language and modal logic, a non-rigid designator is a term that does not extensionally refer to the same object in all possible worlds. For example, consider the phrase "The 43rd president of the United States of America": while the 43rd president of the United States is actually George W. Bush, if things had been different, it might have been Al Gore or Ralph Nader instead. (How remote these possible worlds are from the actual world is a discussion for physics and counterfactualism.) Non-rigid designator are defined by contrast with Saul Kripke's notion of a rigid designator, which pick out the same thing uniquely in every possible world; while there are possible worlds in which the 43rd president of the United States is Al Gore instead of George W. Bush, there are no possible worlds where George W. Bush is anyone other than the man who, in fact, he is. (There are worlds where some person other than George W. Bush is named "George W. Bush," but that's neither here nor there.) Kripke uses this apparent asymmetry to argue (in Naming and Necessity) that no definite description (which can designate non-rigidly) can be the meaning of a proper name.
It is worth noting that some philosophers (such as Gareth Evans) have expressed doubt as to whether non-rigid expressions ought to be called designators at all.
For further discussion, see also: rigid designator.
