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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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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, ...
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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?
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Conditional logic - how to apply a conditional with complex antecedent in tableaux?

I'm referring to the conditional logic of C+ as described Graham Priest in "An introduction to non-classical logic" chapter 5, where the strict conditional is enhanced with ceteris paribus, and a ...
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1answer
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A problem with Modal Logic (necessary truths)

I have this problem with Modal Logic that I am not sure I see which propositions in this logic are necessary besides tautological propositions. I mean metaphysically, something is necessary if it's ...
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1answer
143 views

Questions from a new introduction to modal logic by Hughes and Cresswell

I just want to see if I got this correct. Show that the following wff is valid in every seating arrangement: Mp -> (Lq->Mq). Here's my argument: Assume player A sees at least one player B that ...
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1answer
184 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)...
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0answers
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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? ...
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0answers
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What is the difference between Herbrand Logic and Relational Logic or Predicate Logic?

I am learning a course from Stanford University, and it introduces the notion of Herbrand Logic. However in Wikipedia I cannot find a definition specifically for "Herbrand Logic", only for Herbrand ...
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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) ∧ ...
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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 ...
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Is there a symbolic formulation of modal realism?

Is there a symbolic formulation of modal realism, i.e. the doctrines of modal realism captured in some formal system?
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Real impossible worlds

What is the name of a/the metaphysic that affirms the reality of all worlds including impossible worlds? Actualism denies the reality of all non-actual worlds, possibilism affirms the reality of all ...
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How to express that something that exists is possibly necessary in Modal Alethic Logic?

Suppose that A is True (like 1+1=2) , then how to formulate this proposition in Modal Alethic Logic? If A is the case, then it is possible that A is necessarily the case. For example : If I am ...
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Counter-examples in modal logic

I’m getting stuck on counter-examples to formulas in modal logic. I’m now doing the exercises in Girle’s Modal logic and Philosophy, 3.4.1. Let’s take (a), since the answer to that one is given. The ...