My question is about Rigid Designators. I enjoyed reading Kripke a lot, but I find this concept hard to understand. According to Kripke, a rigid designator refers to, or picks up, the same thing in every possible world. But this way of defining, if it is defining at all, rigid designators is too vague for me to understand. Take 'pain' as an example. Since there are many debates over what pain is (that is, is it a illusion, is it purely physical, it is purely mental, or it is mental and physical etc.), how can it still be a rigid designator if we do not even know what it picks up in our actual world? It could be argue that even though we do not know what it picks up in this world as long as it picks up the same thing in every possible world it is still a rigid designator. But indeed, what would guarantee that it could pick up the same thing?

I'm no expert here, but my recollection is that Kripke reminds us/warns us to avoid the following picture: that we somehow glance into all the many possible worlds and have the task of figuring out which items, in those worlds, are designated by our terms. That would be impossible (for more reasons than one!), not least of all for this reason: suppose there's a possible world where Fred (a dark-haired man in the actual world) is a red-haired woman (and differs in many other traits from actual Fred too). How could we possible look at that red-haired woman (etc) and say, "Oh look there's possible Fred!" The whole point of these "possible variations" on Fred would obscure the possibility of identifying Fred by any of his (her) properties in those other worlds ... Rather, Kripke says, we stipulate possible worlds: we have whatever intuitions we have re: what's possible and we get to stipulate that we are speaking of that world which varies from this world in such and such respects. So if we believe it possible for Fred to be a red-haired woman, we may speak of the possible world in which he is a red-haired woman. We don't need to "guarantee" that our term "picks up the same thing" (i.e. Fred): we stipulate it. Things get a little more complicated re: natural kinds, and you make many good points re "pain," and I recall Kripke has an extended discussion of how we have to distinguish cases/worlds where words get used differently from cases where the natural kind itself is possibly different. But I think, ultimately, the point you make in the penultimate sentence is the right one: when we use natural kind terms our intention is to use them rigidly, even when we may not be entirely sure exactly what their actual denotation is. We don't have to worry re: "guaranteeing" cross-world denotation because we stipulate it.

But I'm sure greater experts than I will weigh in shortly too ....

best, ap

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