Rigid designator
In modal logic and the philosophy of language, a term is said to be a rigid designator when it picks out the same thing in all possible worlds in which that thing exists (and picks out nothing in those possible worlds in which it does not exist). Rigid designators are contrasted with non-rigid or flaccid designators, which may pick out different things in different possible worlds.
The notion of rigid designation was first introduced by Saul Kripke in the lectures that became Naming and Necessity, in the course of his argument against descriptivist theories of reference. At the time of Kripke's lectures, the dominant theories of reference in Analytic philosophy (associated with the theories of Frege and Russell) held that the meaning of sentences involving proper names could be analysed by substituting a contextually appropriate description for the name. Thus, for example, Russell famously held that someone who had never met Otto von Bismarck might know of him as the first Chancellor of the German Empire, and if so, his statement that (say) "Bismarck was a ruthless politician" should be analysed as "The first Chancellor of the German Empire was a ruthless politician" (which in turn could be analysed into a series of more basic logical statements according to the method Russell introduced in his theory of definite descriptions). Against the Russellian analysis (and several attempted refinements of it), Kripke argued that such descriptions could not possibly mean the same thing as the name "Bismarck," because (Kripke argued) proper names such as "Bismarck" always designate rigidly, whereas descriptions such as "the first Chancellor of the German Empire" sometimes do not. Thus, for example, it might have been the case that Bismarck died in infancy. If so, he would not have ever satisfied the description "the first Chancellor of the German Empire," and (indeed) someone else probably would have. But that is not to utter the contradictory sentence that it might have been the case that Bismarck died before he became Bismarck, or that someone else probably would have become Bismarck. Kripke argues that the way that proper names work is that when we make statements about what might or might not have been true of Bismarck, we are talking about what might or might not have been true of that particular person in various situations, whereas when we make statements about what might or might not have been true of, say, the first Chancellor of the German Empire we could be talking about what might or might not have been true of whoever would have happened to fill that office in those situations.
The "could" here is important to note: rigid designation is a property of the way terms are used, not a property of the terms themselves, and a phrase such as "the first Chancellor of the German Empire" could be used rigidly (if one were to say, "The first Chancellor of the German Empire was a free agent like everyone else: he could have decided never to go into politics" that would be a statement about what might have become of the person who actually happened to fill that office, not the wildly implausible statement that it could have been the case that whoever first filled the office of Chancellor was someone who decided never to go into politics). What Kripke argues in Naming and Necessity is that there is an important asymmetry between names and definite descriptions here: whereas a definite description could be used rigidly and could be used non-rigidly, a proper name can only be used rigidly (a word that can be used non-rigidly is, in Kripke's lights, something other than a proper name). And thus, Kripke argues, no definite description could provide the meaning of a name (although the rigid use of a definite description could be used to explain who a name refers to—Kripke calls this "fixing the referent" of the name).
In the course of Naming and Necessity, Kripke argues that proper names and certain natural kind terms—including biological taxa and types of natural substances (most famously, "water" and "H2O") are terms that can only designate rigidly. His account of these terms and the way in which they rigidly designate their objects, though hotly contentious, has been deeply influential in Analytic philosophy. Later philosophers, under Kripke's influence, have also suggested that other terms—such as demonstratives and other general kind terms—might also rigidly designate in certain ways.
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