# Questions tagged [modal-logic]

a type of formal logic primarily developed in the 1960s that extends classical propositional and predicate logic to include operators expressing modality

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### Prove the rule that proves X(P) from X(a) preserves derivability in modal system K

I'm trying to solve a problem which asks me to show that the meta-rule defined by deriving X(P) from X(a) preserves derivability (i.e. if ⊢X(a) then ⊢X(P) in modal system K, where a is a sentence ...
526 views

### S5 proof of ⊢◻(◻P→◻Q)∨◻(◻Q→◻P)

I'm trying to construct an S5 proof of ⊢◻(◻P→◻Q)∨◻(◻Q→◻P). I know that ϕ∨ψ is equivalent to ~ϕ→ψ, and so what I'm really trying to derive is ~◻(◻P→◻Q)→◻(◻Q→◻P) (which is equivalent to ◊~(◻P→◻Q)→◻(◻Q→◻...
1 vote
515 views

### Axiomatic proof of ⊢ □P → □◇□P in S4

As the title explains, I'm trying to give an axiomatic proof of ⊢ □P → □◇□P in S4. This is simple to prove in B, but I'm struggling to see how it's done in S4. I'd really appreciate any help you ...
1 vote
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### Modal validity & vagueness

(2nd version to make explicit my implicit assumptions about A, B and C, and the definitions of the non-logical constants "⊂" and "≡".) Intuitively, the following modal argument ...
211 views

### Does ◻(∀x)(Fx ⊃ ◻(E!x & (E!x ⊃ Fx))) entail ◻((∃x)Fx ⊃ ◻(∃x)Fx)?

I was wondering if we can derive ◻((∃x)Fx ⊃ ◻(∃x)Fx) from ◻(∀x)(Fx ⊃ ◻(E!x & (E!x ⊃ Fx)))? (By the way 'E!' is the existence predicate.) I am using the Quantified Free Modal Logic constructed/...
144 views

### What is reflexivity in temporal logic?

According to Stanford Encyclopedia of Philosophy, reflexivity can be defined by : ∀x(x ≺ x) Where ≺ means "Precedes" : For all x, x precedes x But what does it mean for an instant in time x ...
154 views

### Equivalence of strings of modal operators in modal logic

I'm trying to solve a question which asks me to show that for any two finite strings O₁ and O₂ of □s and ◊s, (e.g. □□◊□◊□), that i) if O₁≡O₂ then OO₁≡OO₂ and ii) if O₁≡O₂ then O₁O≡O₂O where O is ...
180 views

### Logical expression that non-existent things don't do anything?

How could we express the idea that "something that doesn't exist cannot possibly do anything" using a logical argument? And if so, it is at all possible to prove that kind of proposition?
1 vote
167 views

### Semantic expressiveness of modal logic

I am wondering how much of the semantic of basic philosophical questions can be expressed by formal arguments in modal logic. Here is one argument I formalised myself: P1 ◇ ∀a, ∃x // GNB(x, a) ∧ C(a)...
217 views

### Validity of a modal argument?

Would you say that the following argument is valid? And if not, why exactly? For all we know, A may be the state of B; What C does is determined by the state of B; Therefore, for all we know, what C ...
291 views

### On that p does not imply necessarily p

Am I right to assume that in no modal logic, whether in K or in a logic where the accessibility relation is specified as either reflexive, symmetrical or transitive, does ”p implies necessarily p” ...
173 views

### How to prove or disprove “Necessarily P entails P” in Kripke Modal Logic?

I'm doing the body of exercises in Priest's Introduction to Non-classical logic, and got weirdly stuck on 2.12 (o). (I'm on Problems 2.12, if anyone has a link to the answers that would also be much ...
375 views

### Is the problem of logical omniscience intractable?

Vincent Hendricks and John Symons notes the following about epistemic logic: Epistemic logic gets its start with the recognition that expressions like ‘knows that’ or ‘believes that’ have systematic ...
1 vote
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### Going from ◊(∃x)◻Fx and ◻(∀x)(Fx --> E!x) to ◊(∃x)◻(Fx & E!x)

Is it a valid inference (in, say, any quantified modal logic S5 system) to go from ◊(∃x)◻Fx and ◻(∀x)(Fx --> E!x) to ◊(∃x)◻(Fx & E!x)? (E!x is the existence predicate, by the way.)
287 views

### How do pragmatists avoid this modal argument against their view of truth?

I am reading Harry Gensler's Introduction to Logic (Routledge, 2002) and doing exercise 1 of 7.3b in the "Basic Modal Logic" chapter. I think I follow the steps. The answer is in the book. ...
112 views

### Propositions that can't be used to distinguish possible worlds

Are there known ways of formalizing the notion of propositions that can't be targeted by counterfactuals in a coherent way? Or of propositions that are outside the scope of the framework in question? ...
52 views

### Logic and resolving a modus tollens

Wasserman presents a common argument against time travel: (P1) If backward time travel were possible, it would be possible to perform a self-defeating act. (P2) It is impossible to perform a self-...
1 vote
103 views

### Justification for axiom OB-RE in deontic logic

Let OB p denote "p is obligatory". Axiom OB-RE is (p ↔ q) → (OB p ↔ OB q). This axiom seems false to me (under the interpretation of obligation). For example, let p denote "don't lie to me" and q ...
157 views

### If I cannot draw a world in a truth tree in modal logic does that mean that no other worlds exist for that statement?

Suppose I have a truth tree for a modal logic statement that is closed or is open but for which I cannot continue drawing worlds, does that mean that there are no other accessible worlds in that modal ...
1 vote
378 views

### What is the counterexample in modal system K for "⬜A ➡A"?

As I understand it from Modal Logic 2.1 - the systems M, B, S4 & S5, I should not be able to show "⬜A ➡A" in modal logic K. The following truth tree seems to confirm this, if I did it correctly: ...
308 views

### Why can one not draw a world in modal logic given necessity?

In Modal Logic 1.2 -- truth trees for K about 1:30 into the video, the presenter said: If you have possibly A then you need to draw an arrow to a new world and derive A in that world because w0 ...
309 views

### What is the justification for closing a whole branch in a modal truth tree diagram if there is a contradiction in only one world?

I am watching "Modal logic 1.2 - truth trees for system K" presented through Kane B's channel. The standard propositional truth tree diagrams are not a problem. Where I am having trouble is opening ...
1 vote
114 views

### Establishing validity of ◊(∃x)◻[(E!x ⊃ Ox) & E!x]/ ∴ (∃x)◻[(E!x ⊃ Ox) & E!x]

I was wondering under which sort of quantified modal logic S5 system would the inference from ◊(∃x)◻[(E!x ⊃ Ox) & E!x] to (∃x)◻[(E!x ⊃ Ox) & E!x] be valid. Would this require a constant domain ...
442 views

### Can possible-but-non-actual objects have accidental properties?

The modal logic I am considering is the "Simplest Quantified Modal Logic" which combines first-order predicate logic with identity, with S5 in the most straightforward way, described here and slightly ...
56 views

### How to translate classical logic to basic Kripkean logic?

I'm trying to semantically prove the following argument (sorry about the formatting - I'm new to stackexchange): if P ⊨c Q then ⊨k 򪪪(P → Q) However, I don't know how to translate or relate the ...
240 views

### How to denote physical (not logical) impossibility in modal logic?

Say I want to note that in this world, it's physically impossible to jump over the Eiffel Tower. I can just write ¬◇x, but this seems to say that in all possible worlds x is impossible, and I want to ...
204 views

### Why does modal logic need to use ◻◻p?

In Frederic Fitch's Symbolic Logic he proves (11.8, page 66) that "◻p" coimplicates "◻◻p". In 11.10 (page 66), he writes, A system almost the same as the system Lewis calls S2 is obtainable by ...
1k views

### Does anyone have a proof checker they prefer using for modal logic?

I am looking for a proof checker for modal logic using natural deduction or sequent calculus. I am trying to learn Isabelle, but I think there should be a simpler solution. Although I can rely on ...
435 views

### What's the difference between logical modalities and physical modalities?

I am just wondering what's the difference between the two. I would say that there is something different, but honestly I can't define what it is exactly. What do you think?
381 views

### What is Quine's perspective on probability?

I am curious about what Quine's perspective on probability may be and if we can say that the quinean viewpoint on modality can be considered similar to his viewpoint on probability. Is probability ...
1 vote
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### Verum or T in dynamic epistemic logic models

I've been reading a paper on dynamic epistemic logic where they use T in a way that I'm not really familar with. The paper is here by Wesley Holliday, page 16: https://pdfs.semanticscholar.org/dae6/...
321 views

### What logic does Fitch's paradox use?

So I have been looking into Modal Logic as well as epistemic logic (and its dynamic versions) with the hopes of studying their applications to Fitch's paradox. Fitches paradox refers to the proof ...
1 vote
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### Need help formally translating sentence into modal logic

Can someone please help me formally translate the sentence "it is impossible for x to exist without y existing as well" into modal logic using ~◇? Or, similarly, the sentence "it is impossible for ...
350 views

### Axioms for modal logics based upon counterfactuals

Suppose we have a logic for counterfactuals as with David Lewis. I here use ⇛ for the counterfactual conditional. So suppose we have: Rules: (1) If A and A→B are theorems, then B is a theorem. (2)...
997 views

### How does the concept of the 'virtual' (Deleuze) relate to 'counterfactuals' (Lewis)?

We read: "[...] Deleuze will reject the notion of the possible in favor of that of the virtual. Rather than awaiting realization, the virtual is fully real; what happens in genesis is that the ...
1 vote
81 views

### Does Sosa's modal definition of knowledge beg the question?

In his 1999 paper "How to Defeat Opposition to Moore", Ernest Sosa argues that sceptical, Nozickian tracking, relevant-alternative, and contextualist accounts of the sceptical paradox rely on the ...
263 views

### How to construct a counter-model of □P --> □◊P in T and K?

I am new to modal logics and would really appreciate it if someone would be able to help me out with this practice question. I’ve established the validity theorem in T with an axiomatic proof, but I ...
77 views

### "Fatalism and Professor Taylor" by Bruce Aune

In this paper Bruce Aune states the following Taylor would probably want to maintain that (p ≺ q) ⊃ — (p → q), where "→" represents physical, or natural, implication So I'm not clear what "...
167 views

### Negation of a E quantifier in a Modal Logic T System?

In System T its possible to conclude: (1) □Ex->Ex Therefore its possible to conclude: (2) Ex->◊Ex But whats the result, if I negate the existence quantifier? Is it: (3) ~Ex->~◊Ex or is it: (4) ~Ex->◊~...
229 views

### How many equivalence classes does the accessiblity relation have in S5?

I'm a math student taking philosophy classes. So I have some math background but am absolutely unfamiliar with metaphysics&c. Recently I've come around the "Nothing Is Impossible" paper as a part ...
525 views

### Natural deduction introduction and elimination rules for modal quantifiers

I've read several papers on modal natural deduction but I've only been able to find one clear explanation of []-Introduction ([]A if A can be proved from no assumptions/premises). But there were no ...
257 views

### In possible-world semantics, what do nested modal quantifiers mean?

I'm trying to learn modal logic and I'm having trouble translating sentences like [][]A and []<>A into natural language using possible-world semantics. The first statement seems to read, "In ...
171 views

### circularity of possibility defined over possible worlds

I have just checked similar questions that have been asked(and answered) before but I am still confused. How is it that the notion of possibility defined over as being True in some possible world not ...
97 views

### introduction to modal logic

I am not entirely sure if we are allowed to ask such questions here but I was wondering if anyone could suggest me a real introductory level textbook on modal logic
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### How to valuate (assign truth values to) a formula in modal logic (Kripke model)

A total novice here, and exasperated at that. I can translate natural language statements into formulae of modal logic, but their valuation in Kripke model seems elusive, as I'm simply unsure how to ...
689 views

### The difference between indicative conditional and counterfactual

I was confused about the difference between indicative conditionals and counterfactuals. Could someone please offer an elaboration?
1 vote
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### Why isn't 4 a theorem of K? and Can I use Show boxes in modal logic?

In my basic logic course if I want to prove A ⊃ B then I would do the following: 1 Show A ⊃ B 2 A Assumption for Conditional derivation. 3 Stuff..... 4 Show B 5 More stuff.... 6 B [or some ...
1 vote
347 views

### Are all logically necessary statements self identity claims?

When we say that x is necessarily P, are we not asserting that this is the case regardless of all contingent facts, so regardless of what x and P are? And if x was P regardless of what x and P are, ...