# 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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### Nonexistence and invalid formulas in modal logic

In first-order logic, I can essentially just ignore issues related to nonexistence and invalid formulas, without losing much. There is also free logic, in case I'm not happy with simply ignoring these ...
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### Possibly, 7 isn't prime [closed]

I'm curious if in the philosophy of mathematics (or perhaps the philosophy of modality), the following has been proposed: There exists something like an imaginary (but not complex) number i such ...
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### What determines accessibility of possible worlds?

Recently, I have begun studying modal logic, using Brian Chellas's Modal Logic: An Introduction. Something keeping me from fully understanding the material is the idea of a possible world. They seem ...
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### Reasoning in S5

I'm currently working on implementing reasoning involving time. Since S5 (every world accessible from any other) is sufficient for what I'm trying to represent, I wanted to know what are the ...
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### One-placed intersection operator

What is the meaning of a formulation like: "A iff ∩A ⊆p" A is a set of propositions, p is a specific proposition, and the whole formulation is explicated as "There is no possible world where all ...
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### For the modal realist, do possible individuals (and worlds) exist necessarily?

For David Lewis's Modal Realism, do the worlds and individuals that inhabit them exist necessarily? In a sense, the answer is "no". For an individual to exist necessarily would be for it to have a ...
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### Are there mathematical properties a mathematical object might have only contingently?

It is generally assumed that mathematics is necessary, such that any mathematical theorem is necessarily true. This can be read as a de dicto necessity such that for any mathematical proposition p, []...
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### Modal Logic: a question concerning accessibility

I’m reading a lot about modal logic lately, right now Lewis “On the Plurality of Worlds” and Priests “Introduction to Non-classical Logics”. It is postulated that the different worlds have nothing to ...
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### Does modal realism dispense with Occams razor or embodies it?

Modal realism posits the reality of all logically possible worlds. This seems to radically dispense with Occams razor by allowing the reality of all logically feasible explanations. But on reflection ...
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