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It is often useful to interpret statements in various modal logics using possible-world semantics. For instance "it is necessary that P" means "P is true in all possible worlds", "it is possible that P" means "P is true in some possible world", "it ought to be the case that P" means "P is true in the best possible world", "if P were true then Q would be true" means "in the closest possible world in which P is true, Q is true", etc. There are two ways to interpret these sorts of statements. Modal realists like David Lewis believe that modal statements are about real parallel universes that exist and which we may even be able to travel to in the future. Modal fictionalizers believe that possible worlds are just a useful fiction for understanding modal statements, and that "there exists a possible world in which gravity is a repulsive force" is only true in the sense that "Sherlock Holmes lives at 221B Baker Street" is true, i.e. true within its fictional context.

But my question is, is there anyone who advocates a third position, where all modal statements are trivial or meaningless? That is, someone who agrees with modal realism that modal statements really are about possible worlds, but also believes that the actual world is the only possible world. Such a person would believe that "it is necessary that P", "it is possible that P", and "it ought to be the case that P" all reduce to "P is true", and that the counterfactual conditional "if P had been true then Q would be true" reduces to the material conditional "not P or Q".

Such a position would eliminate large parts of human discourse and thought, including morality, causation, possibility and necessity, knowledge, etc. But I'd be curious to see if someone has made an argument for doing so.

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    Willard van Orman Quine. You can see e.g. Dagfinn Føllesdal, Quine on Modality (1968) as well as W.V. Quine, The Problem of Interpreting Modal Logic (1947). – Mauro ALLEGRANZA Nov 15 '16 at 9:02
  • Formally, if your logic doesn't admit modal operators, then using a modal is meaningless. That is, if all you have is truth value, 'necessity' is meaningless. So base statements like "Eating people is wrong" don't have truth values. But truth isn't everything. – Mitch Nov 15 '16 at 18:12
  • Radical modal nihilism is only used as a straw man, rather than "eliminate large parts of human discourse" it is shrewder to explain them away as a roundabout talk about the actual. In this spirit see actualism and modal skepticism. Here is the gist from Quine in Pursuit of Truth:"In its everyday use as I described it, 'necessarily' is a second-order annotation to the effect that its sentence is deemed true by all concerned, at least for the sake and space of the argument". – Conifold Nov 16 '16 at 2:47
  • philosophy.stackexchange.com/questions/33685/… may be of some interest, idk – user6917 Dec 15 '16 at 22:01
  • By the way, apropos of nothing, it is possible that gravity is a repulsive force in this universe. One French physicist (I think it was) even proposed a theory. – PeterJ Jan 10 '18 at 10:17
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Even in Quine's understanding, I wouldn't call them trivial or meaningless, but incomplete and therefore vacuously true.

Every 'should' statement, for instance, is true for someone crazy enough to believe it: 'You should kill anyone who owns a ShiTzu' is true for someone who worships the great ShiTzu-avenging godling of New Berlin. If you develop an acute schizophrenia tomorrow, that person might be you. (He might be you now, so let me inform you that I do not own a ShiTzu.)

Such a statement can only become false when a given moral principle is attached afterwards as a premise or tacitly presumed by the speaker; say, religious relativism, or obeying the law. At the point the omitted condition is adequately grounded, the statement is rendered conditional and not modal.

This remains a position other than realism or fictionalism: since this clarification is ultimately intended, direct modal statements are statements about reality, this reality rather than some extended one, and they not fictions. (That does not mean that modal statements about fictions are not about fictions. But we make indicative statements about fictions, too.)

The same is true of the statement's inverse -- they are both true, and do not conflict. There is no problem with 'You should and you should not do X.' It simply indicates your moral premises are not clear enough yet. So the mechanism of box and diamond operators in modal logic just tracks what is and is not grounded yet, relative to a single given missing premise, and therefore cannot lead to broader deduction. This makes it a much less useful tool than it might seem.

The notion of moods captures the fact that humans never really intend to clean up after themselves in this regard. We just rate the odds of guessing the condition wrong, and continue. The mood marker (would, should, could, etc.) besides conveying this intention to remain elliptical, indicates the likely form of the condition. But it does not really have meaning in and of itself, and does not even prescribe the condition's nature entirely. One can clarify what initially appears to be a moral issue by providing facts, for instance, which displace the applicability of some given premise other than the missing one(s).

Grammatically this interpretation of modal statements as vacuously true due to missing premises is now called 'suspending control', and is one modern interpretation of the grammar of the subjunctive and related constructions. In this model, when you use subjugation with a mood like the classical English subjunctive or the old Greek 'optative', you are making the statement for which a condition exists which controls the meaning of your statement, but you are admitting that you are not certain of exactly what that condition is.

This whole framing makes modality elliptical, rather than nonsense.

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