13 votes

What does □□p mean?

In general □□ p and □ p are very different. Thinking in terms of Kripke frames, they only obviously coincide if the accessibility relation is transitive. This is true for Kripke frames validating S5, ...
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11 votes
Accepted

Is it consistent to say "X is possible but false"?

Well, in English you would use the subjunctive tense and say, "possibly I could have had a cat in my room, but in fact I do not." That's a reasonable statement to make. "I could have ...
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  • 6,297
10 votes

What is the difference between a probability and a possibility?

On the prevailing extensional interpretation of modality the difference between possibility and probability is the diffference between quality and quantity, possibility is the quality quantified by ...
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  • 40.7k
9 votes
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Is the problem of logical omniscience intractable?

Logical omniscience was always only a technical problem related to formalization of epistemic logic in terms of possible worlds. Since classical possible worlds are supposed to be consistent and ...
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  • 40.7k
9 votes

How to be skeptical of transcendental arguments?

I don't see any need to be 'sceptical' of transcendental arguments. It's rather like being sceptical of deduction whether done classically or modally. And in fact, a transcendental argument relies on ...
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8 votes

Why did many valued logic fail in describing modal logic?

So the three valued logic of Łukasiewicz has three truth values {1,i,0}. Łukasiewicz was trying to solve the problem of future contigents with this logic. His view is that statements about the past ...
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  • 842
8 votes

Does this modal ontological argument prove the existence of God?

I understand your argument as follows: A maximally great being possibly exists. If a maximally great being possibly exists then it necessarily exists. If a maximally great being necessarily ...
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  • 6,445
8 votes

Should truth entail possible truth?

If we're talking about metaphysical possibility, then normally yes. If you reject the claim that "if P then possibly P", you must also reject the claim that "if necessarily P then P". Proof: suppose ...
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  • 3,759
8 votes
Accepted

Is every world accessible to itself?

You are correct about the relationship between □P → P and the reflexivity of the accessibility relation. As to whether you want to take this as an axiom, it depends entirely on your intended ...
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  • 15.5k
7 votes

What is the explanation for inferring existence in every possible world from existence in some in Plantinga's Ontological Argument?

This looks like a "lame terms" redux of Plantinga's "victorious" ontological argument from The Nature of Necessity. Here is Plantinga's explanation of why if a maximally great ...
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  • 40.7k
7 votes
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How does the concept of the 'virtual' (Deleuze) relate to 'counterfactuals' (Lewis)?

In a sense, Deleuze's virtual and Lewis's possible worlds compete to provide the "right" conception of the possible. The descriptions are indeed similar but this is deceptive, Deleuze and ...
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  • 40.7k
7 votes
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How to prove the completeness of S5?

Hughes and Cresswell's proof proceeds roughly as follows: They show that any sentence of S5 is logically equivalent to a modal conjunctive normal form (MCNF) in which a sentence takes the form of a ...
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  • 15.5k
7 votes

How to be skeptical of transcendental arguments?

The argument form that you quote is straightforwardly valid in standard logic. It might be symbolised as 1. ◇Y → X 2. Y 3. Y → ◇Y therefore, 4. X You give 1 and 2 as premises, and 3 is the T axiom ...
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  • 15.5k
6 votes
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What are the prerequisites for studying modal logic?

An understanding of classical propositional logic and first-order logic should suffice. Some notes: Modal languages look very much like non-modal ones. For example, if you have a non-modal ...
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6 votes

What's a good source for refreshing my formal logic skills prior to graduate school?

Peter Smith—who is or has been a user of this forum—has a discussion article posted called Teach Yourself Logic 2015: A Study Guide (PDF, iv + 94 pp. Last updated 1 Jan 2015). It's on his website. It ...
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  • 5,398
6 votes

Why did many valued logic fail in describing modal logic?

The specific quotation you gave about Łukasiewicz refers to the fact there was an attempt to understand intuitionistic logic as a many-valued logic, but this failed because Gödel proved in the early ...
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  • 15.5k
6 votes

Does this modal ontological argument prove the existence of God?

The proof does not work, I think. Your problems are premises 1 and 2. My main problem is with premise 1: it is possible for a maximally great being to exist. Why should we accept this? There is the ...
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  • 1,914
6 votes

What is Plantinga's "trivial property"?

Plantinga uses the concept of non-trivial properties in his transworld depravity defense of God's benevolence, see How does free will defense of God's benevolence work? Ciprotti in Theological ...
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  • 40.7k
6 votes

What does □□p mean?

To add to the existing answer, there's also epistemic logic, where □ is interpreted as knowledge (relative to a given subject). Possible worlds in such a system are worlds that are consistent with ...
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  • 6,445
5 votes
Accepted

How does Plantinga's free will defense of God's benevolence work?

Geirsson and Losonsky, in Plantinga and the Problem of Evil (from your "P.S." statement) fail at least two ways in attempting to address Plantinga's argument: Counter-Factuals (CFs) are possible ...
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  • 956
5 votes

Does necessary possibility entail actuality?

This is a 2D variation on Sequitur's already complete answer. The question is whether: 1) ▢♢φ |= L Aφ holds for modal logic L. The answer depends on which logic this L is. ...
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5 votes

Does necessary possibility entail actuality?

The entailment does not hold in the modal logic S5 and so it also doesn't hold in all weaker normal modal logics (normal in the sense that they validate the K-Axiom □(A→B) → (□A→□B)). Let a (...
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  • 1,388
5 votes
Accepted

What exactly is metaphysical possibility?

Some formal notes to complement Mauro's excellent answer. As one would expect in a discussion of modality, we're going to talk about modal models when defining things. Most will be familiar with ...
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5 votes

Is Modal Logic Logic?

Here's an argument from Aristotle which I'm paraphrasing: If we ought to philosophise, we ought to philosophise If we ought not to philosophise, then we ought to philosophise in any case, we ought to ...
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5 votes
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Is it possible to not know that one knows p?

Everything hinges on how you understand "knowing." First off, you're going to need to decide whether there's a distinction between episteme, techne, and phronesis (or if you prefer Gilbert ...
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5 votes
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Why do “L” and “M” name the strong and weak modal operators in modal logic?

From the Polish logician and philosopher Jan Łukasiewicz who invented the Polish notation for logic (named after his nationality) : M ϕ for możliwość : possibility L ϕ for konieczność : necessity [...
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5 votes

Does this modal ontological argument prove the existence of God?

The proof fails in the very first step. You try to justify this step in your initial explanation, but let's just put this explanation as part of the proof. Nobody has successfully disproven the ...
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5 votes

Is □(□A→B) v □(□B→A) provable in S5? If it's not, I am having trouble coming up with a countermodel

This statement is provable. You can turn it into an equivalent implication: ¬□(□A → B) → □(□B → A) Then make the hypothesis that the first term is true and show that the second follows. The first ...
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5 votes

Should truth entail possible truth?

Obviously truth implies possibility. So let me make a case for truth not implying possibility. Let's start with an "applied logic" example. Suppose I'm trying to reason about the world using ...
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5 votes
Accepted

What are good sources on vivid designators?

Vivid designator (originally "vivid name") is Kaplan's replacement for rigid designator in the logic of beliefs and other propositional attitudes introduced in Quantifying In. The point was ...
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