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Semantic externalism, or more broadly externalism about mental content, is the idea that propositions, intentions, attitudes, etc. derive their meaning from the external world as opposed to internally from the mind itself. Some of the most famous arguments for this view are Putnam's twin Earth thought experiment and Davidson's swampman thought experiment.

The most common, and widely accepted as being the most philosophically well founded, semantics of modal statements are possible worlds semantics which say that a modal statement, either something about necessity or possibility, derive their meaning from a set of possible worlds. If a proposition is true in all possible worlds then it is necessary, if it is true is some possible worlds then it is possible, and if it is true in no possible worlds then it is impossible.

David Lewis was a proponent of modal realism, the idea that possible worlds are all physically real. He argues that in a statement that begins with "In the real world," "real" in this context in just indexical and refers to our own world. Saul Kripke, who put down the foundation for modal logic with possible world semantics, rejected modal realism and argued that possible worlds are absolutely not physically realized but are instead just linguistic and philosophical tools.

  • If a philosopher accepts semantic externalism and accepts that possible worlds are the correct form of semantics for modal statements, is she then forced to commit to a belief in modal realism?

  • If those two assumptions aren't strong enough to lead directly to modal realism by themselves, what happens if she also accepts a general metaphysical realism?

My assumption is that there is no way to get around modal realism if you believe (1) external things are real, (2) external things are what give propositions meaning, and (3) possible world semantics are what give modal statements meaning. Most of all, I am interested in any papers that deal with this question.

  • Technically handy, yes, "philosophically well founded", I doubt it: "philosophers reading this will irritably exclaim that these assumptions are distorting oversimplifications which they do not make when applying possible world semantics to their concerns. However these are assumptions that have to be made if the techniques of possible world semantics are to be applied, and if these assumptions are wrong then the point is being conceded that possible world semantics are not applicable to their concerns." Forster, Modal Aether – Conifold Jan 30 '17 at 20:07
  • @Conifold the "most" extended to "common" as well as "accepted", possible world semantics are the most widely accepted to be philosophically well founded explanation of modal semantics. At any rate, even if you still disagree with that statement it doesn't either (1) answer my question or (2) explain why the question isn't a correct thing to ask. – Not_Here Jan 30 '17 at 20:10
  • Considering that Kripke is Putnam's partner in externalism I am guessing that the answer is no. But I was curious myself how Putnam managed to reconcile his externalism with his (eventual) rejection of realism, How does Putnam reconcile having referents in language with rejection of realism? So I have nothing against the question, it's just best not to pass vague and controversial at best claims as fact ("most accepted" by whom?). – Conifold Jan 30 '17 at 20:26
  • @Conifold Semantics by Kate Kearns has an entire chapter dedicated to modality, and it exclusively deals with possible world semantics. What is Meaning?: Fundamentals of Formal Semantics by Paul H. Portner has a chapter that deals exclusively with modal semantics, possible worlds and accessibility relations. – Not_Here Jan 30 '17 at 20:46
  • From Portner: "There is an enormous literature on modality, and much of it has to do with refining the theory of accessibility relations. This gives rise to more sophisticated classifications of modals.” Any sort of inspection of papers and textbooks on semantics shows that they deal almost exclusively with possible world semantics when discussing modality. The answer to “most accepted by whom” is “the vast majority of professionals writing papers and textbooks on the subject.” – Not_Here Jan 30 '17 at 20:46
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This question seems to have been resurrected; I trust no-one minds if I answer it.

If a philosopher accepts semantic externalism and accepts that possible worlds are the correct form of semantics for modal statements, is she then forced to commit to a belief in modal realism?

No, at least not the extreme modal realism of the sort Lewis endorses. Here's a rough-and-ready view that accepts a possible worlds semantics for modals but denies Lewisian modal realism:

  1. Well-formed sentences in a language express propositions.
  2. Propositions are complete, maximally consistent sets of possible worlds.
  3. Possible worlds are abstract objects.
  4. A sentence S is true at a world w iff w is a member of the proposition expressed by S.

This view obviously avoids Lewisian modal realism. Even if 'external things' exist and are the semantic values of certain expressions (I trust this is what you mean when you say that "external things are what give propositions meaning"), we're not thereby committed to thinking that the external things in question are Lewisian possible worlds. They could simply be abstract objects like numbers or sets or universals. Of course, this view is still a 'modal realist' view insofar as it accepts that possible worlds exist; but it departs from Lewis when it comes to their nature. (Could you be an anti-realist about possible worlds yet go in for possible worlds semantics? Maybe. But you'd probably have to be some sort of error-theorist or non-cognitivist about modal expressions. I don't know of any particular views in this neighbourhood, or why you'd be motivated to accept them).

Does anyone hold this kind of view? Yes; Robert Stalnaker comes to mind. And similar sorts of views are clearly possible, each with their own tweaks - the literature on philosophical semantics is replete with them.

Now, it's worth pointing out the Lewisian will deny that you can correctly capture the truth-conditions of all the sentences you want to capture on the above view. He's going to say that you need possible worlds, in his sense, to do semantics. The Stalnakerian, say, will disagree; the Stalnakerian thinks we can do the semantics we want without committing ourselves to Lewisian possible worlds. None of this should be surprising; whether or not modal semantics need Lewisian worlds is something of an open question. The point is that philosophers are very resistant to Lewis's conception of possible worlds and have gone to great lengths to preserve the benefits of the possible worlds semantics while eschewing Lewisian worlds.

As far as references go, you should really check out Robert Stalnaker's work. He's been writing about this stuff for ages. In particular, check out the first part of Stalnaker's Ways a World Might Be as well as his Mere Possibilities. The third chapter of Ways a World Might Be contains a very nice little dialogue between a Lewisian and an anti-Lewisian that you might find helpful.

If those two assumptions aren't strong enough to lead directly to modal realism by themselves, what happens if she also accepts a general metaphysical realism?

Nothing, as far as I can tell. Stalnaker, for instance, is a metaphysical realist but an anti-Lewisian. Rather, to get from semantic externalism and possible worlds semantics to Lewisian possible worlds you need the premise that the only or at least the best way to correctly capture the semantics of modals is with Lewisian possible worlds.

I hope I've managed to track your question and what's bothering you. If not, please let me know and I'll take another stab at it.

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    No issues with the answer, thank you for the references. To be honest, at the time I had a feeling Stalnaker was what I needed to look at but for disparate reasons I had to focus on other things and forgot about this question. I'll go find a copy of his book and try to remember what it was exactly that was bothering me – Not_Here Jun 10 '17 at 16:16

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