# Questions tagged [truth-tree]

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### I think I finally understand alethic nihilism

So the definition of truth is that p is true if and only if p. However, since the Liar Paradox states that p is true if and only if p is false, this is a contradiction. So the words truth and falsity ...
• 151
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### How Does Alethic Nihlism Address The "Changing Of Subject" Objection?

I recently read this article in my pursuit to understand wtf alethic nihlism is trying to say: https://philarchive.org/archive/ASASIT One of the objections to alethic nihlism is that alethic nihlism ...
• 151
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### Alethic Nihlism

I read this article: https://aeon.co/essays/on-the-advantages-of-believing-that-nothing-is-true I just don't understand how someone can believe something is the case but is not true. It's like saying ...
• 151
118 views

### How to generate counterexamples for modal logic trees from Girle

I am having a heck of a time understanding a counterexample discussion in Girle 2000. Here is the tree: But then, on p. 40, Girle reads off the tree: I understand where the values in k and l come ...
• 41
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### Some Questions About A Truth Tree

The logical system used in this post is first-order logic. I’ve been reading Introduction to Logic: Predicate Logic, 2nd edition by Howard Pospesel, and I have some questions concerning a truth tree. ...
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### Proof Tree to Fitch Proof

I was wondering if anyone could help me on a proof I've been working on: I was able to check that it is valid with a proof tree generator (prooftools): However, I still haven't figured out the proof....
1 vote
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### Prove that if the tree method determines that a set of sentences T implies a sentence A, then T does in fact imply A

Having trouble wrapping my head around how to prove this. My first question about this is what it means for the tree method to determine that the set of sentences implies A. I'm taking it to mean that ...
• 19
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### How does a truth tree provide positive and negative effect tests for implication?

I'm trying to prove that the truth-tree method can be used to give a positive effect test for implication, and a negative effect test for non-implication. I've been given the fact that The truth-tree ...
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 ...
• 19.2k
1 vote
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### 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: ...
• 19.2k
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 ...
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