Questions tagged [logic]

For questions about logic, whether it concerns syllogistic logic, mathematical logic or the nature of logic itself.

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Why isn't humanistic logic taught in schools any more?

Specifically, I'm curious about the loci or categories that Agricola and later Ramus used extensively. Were they found to be problematic at a later time? If not, why not use them? They're so helpful ...
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What is the logical distinction between “the same” and “equal to?”

We all understand that Given A = C, and B = C, Then A = B. However, A is not “the same as” B. Example: A is the question, “What animals have feathers and can fly?” B is the question, “What was the ...
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Is it reasonable to arrive at a conclusion based on lack of evidence to the contrary?

I was having a discussion last night about my atheism, and it was suggested to me that agnostism is the only reasonable conclusion, as it leaves the door open for new information. This made me think, ...
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A bizarre question asked at a job interview about conditional probability [closed]

I was recently asked a "logic puzzle" type question at a job interview which seems to be a slightly mangled version of a N-coin puzzle (intended to test the subject's knowledge of conditional ...
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Are all non self-referential statements true or false?

It is well known that there exists self-referential statements which are neither true or false, such as "I am lying". Is it possible to have statements neither true or false which are not self-...
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If the universe is infinite, shouldn't I already have been contacted by a time and space travelling doppelgänger?

If the universe is infinite, by virtue of chance it means that every possible configuration of matter must exist somewhere (according to this documentary). Therefore, if the universe is infinite and ...
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What is the difference between the “is” of predication and the “is” of identity?

What is the difference between these, the "is" of predication and the "is" of identity? For example, when I say, "my pet is a cat", am I using "is" as an identity or as a predicate?
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Why are homologies evidence for evolution instead of common design?

I have seen some creationists arguing that when evolutionary biologists use homologies (anatomical or genetic) as evidence for evolution, they are committing the fallacy of affirming the consequent. ...
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What is the difference between logic and reasoning?

What is the difference between logic and reasoning? I can makes sense of what logic is about. But when it comes to reasoning what does it do more than logic does? Can you give an example which shows ...
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Is it possible to be truly unbiased?

I was pondering this question: Inductive Argument Against Believing Anything I his argument, the OP suggests that, in the position of choosing a system of belief where there are multiple, large ...
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Why might you not accept ¬(¬A) = A?

What motivates intuitionism's rejection of double negation: If A exists, then ¬(¬A) = A. I can't see what's wrong this statement or why someone would reject it.
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is “2+3=6, then 2+3+1=7” an example of the fallacy fallacy?

Is "if 2+3=6, then 2+3+1=7" an example of a fallacy fallacy or argumentum ad logicam? I believe it is an example of the fallacy fallacy, but I'm not able to show it. I only think it is a fallacy ...
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Are truth-claims inappropriate?

Strictly speaking, is it inappropriate to make a truth-claim? I am seeking an answer from Philosophy (Epistemology), and feel free to use logic I am speaking "theoretically", not "practically" I am ...
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Is there a name for each individual's perceived sphere of reality?

Is it an acceptable idea that each individual carries their own model of reality in their mind? Is there a name for the model that each individual uses to perceive reality? Is there a name for the ...
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Does the Fallacy Fallacy make logic useless?

I should add that I'm not a formal student of philosophy and haven't studied it in any serious depth. I just like logic, and logical fallacies. I like to spot them, and I like to debate using them, ...
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What does it mean for an axiom to be logical?

I have recently been hearing the phrase logical axiom being thrown around in reference to the philosophy of mathematics and I'm having a hard time understanding what one might mean when they are using ...
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Asymmetries in two opposite arguments from ignorance

Joe claims: "There is no proof that unicorns exist, therefore unicorns do not exist". Alice claims: "There is no proof that unicorns do not exist, therefore unicorns exist". Bob claims: "There is no ...
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What is the meaning of “predicate” in this definition?

I have trouble understanding the article on existence in this Philosophy dictionary. Instantiation in reality, or actual being. Kant pointed out that existence is not a predicate. What is the ...
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What is the philosophical problem with Skolem's Paradox?

I guess there are two questions here. QUESTION 1: Skolem's Paradox shows that countability is relative in first-order logic, but where is the relativity? In this first question, I will do the ...
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What did Gödel mean by “positive property” in his ontological argument?

In his ontological proof, Gödel states (Axiom 1) If a property is positive, then its negation is not positive. What does he meant by this term? I have come across authors who replace this notion ...
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What is the difference between identity and equivalence?

While the problems of identity seem to have more heft in philosophy, I am actually more interested in the meaning of "equivalence" as symbolized in the (=) sign and, in guilt by association,with ...
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What logical systems categorize A->~A as a contradiction.?

In the basic propositional logic I learned in school A->~A is not a contradiction because it is not false when ~A is true. What logical systems would hold this statement to always be false?
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Are “If P then Q” and “Q only if P” equivalent?

My textbook says "If P then Q" and "Q only if P" are logically equivalent, but consider this: "If it's green, it's poisonous." "It's poisonous only if it's green." Now say there's a purple ...
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How does one prove De Morgan's laws for quantifiers?

One of De Morgan's laws state that ¬∃x P(x) is equivalent to ∀x ¬P(x), but how would one go about formally proving this? Numerous attempts to find a solution have been futile, even proofwiki.org ...
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Do all objects have properties?

Assumption: An object has no properties. I can think of that object as an object with the property that it has no properties, but this is a contradiction to my assumption. Hence all thinkable objects ...
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Can we do without Necessity?

It appears that necessity is a dead idea that, unfortunately, only philosophers still gush over. The association of epistemological and logical necessity with ontological necessity ran its course ...
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Is “Don't blame me; I voted for ___” a bad argument?

Is there a fallacy in the argument, "Don't blame me; I voted for ..."? Or is a voter's entire responsibility for their contribution to whatever current state of political affairs they experience ...
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Logical fallacy classification: comparing the best from one group with the worst in another

I am looking for the name for the logical fallacy that compares the best samples from one group with the worst samples from the second group and concludes that the first group is better than the ...
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What's a good source for refreshing my formal logic skills prior to graduate school?

I need to flex my formal logic muscles prior to graduate school--I've had a dry spell in my logic practices while finishing my mathematics degree, particularly since the logic used in analysis, ...
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How does “higher-order logic” differ from “normal” (first order?) predicate logic?

How does “higher-order logic” differ from “normal” predicate logic? I assume the latter is consistently called ”first order logic”. So where are the differences between these? What kinds of statements ...
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Existential import: in logic, do propositions default to true or false when objects in them do not exist?

In this hypothetical: Firefighters always tell the truth, while politicians always tell lies. Suppose three people, who are either a mix of firefighters and politicians, all politicians, or all ...
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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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How does Logic define “true” and “false”?

"Truth", "Falsehood" are pretty axiomatic expressions, but even axioms need to be defined in common language terms. What are the "official" definitions of these in Informal logic, Formal logic, ...
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What is an 'argumentum ad lapidem'? [closed]

I've seen philosophical arguments descriped as 'argumentum ad lapidem', but never quite understood what the criticism is trying to indicate. As best I can tell, it's a dismissal of an argument with ...
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Does an English translation of “Logica ‘ingredientibus’” by Peter Abelard exist?

Pretty much the title, first time I'm having trouble finding anything.
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Logic and Computation: a philosophical viewpoint on Curry-Howard isomorphism

The link between logic and computation is stronger than ever, especially since the establishment of the Curry-Howard isomorphism specifying that proofs can be seen as programs and formulas as program'...
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Can an eternal god destroy himself? [duplicate]

Possible Duplicate: Is the definition of God consistent? (primary) also: An immovable object and an irresistible force God's paradoxes and their implications To many, at first sight, ...
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Is Ross' paradox really a paradox?

Consider the following disjunction introduction: Your room is clean. Therefore, your room is clean or your house is burnt down. Ross' paradox allegedly arises when applying this inference to ...
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Introductory book on philosophy of logic?

I know that there are quite a few questions like this here already, but I haven't yet found an answer that would satisfy me. I'm looking for an introductory logic book. My main goal is to work ...
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What name or label applies to a statement the very act of making which contradicts its truth?

Consider a proposition stating something to be true that by the very act of stating denies truth to the speaker. This is not a fallacy, which is an argument incorrect by virtue of its form. But I am ...
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Is there an onus on the critic to provide an alternative solution?

In many situations, when someone criticises a procedure, or a policy, a common response is something like "yeah, we know it's crap, but you're not presenting an alternative solution, so shut up". ...
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Are there any books which reduce famous Philosophical Arguments to Logical Notation?

I have just started studying logic and wish to find some example arguments by famous philosophers broken down very plainly into their premises and conclusions, along with the logical notation to the ...
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Desirable features in an introductory logic textbook?

I have been very unsatisfied with the quality of the free, open-source textbooks in logic I have found so far. Some, like P. D. Magnus's forall x and the wikibooks Formal Logic text seem overly ...
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How many people can know a secret? [closed]

I'm not sure if this is a question with a clear answer, and if this is the correct stack to ask it, but is there a limit for how many people can know a secret? An entire goverment can keep a secret, ...
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Is formal logic a branch of philosophy?

Pondering over logic in the everyday 'ways of reasoning' sense of the word, classifying informal logical fallacies, talking about kinds of arguments and things like this surely belong to philosophy. ...
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In what way is the lambda calculus stronger than first-order logic?

This wikipedia article on combinatory logic says that Combinatory Logic, Lambda Logic and Turing Machines are equivalent computationally, but that both of these exceed the expressive power of first-...
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Which ontological commitments are embedded in a straightforward Turing machine model?

Let's assume that in response to a question or problem, a certain type of idealized finite Turing machine can be presented as an answer as if it would really (=physically) exist. Is the corresponding ...
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What's the point of 'dthat'?

David Kaplan famously formulated a logic for demonstratives (including terms like 'I', 'now', 'here' 'actually' etc.), LD, which is a version of first-order two-dimensional logic. Very roughly and ...
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Difference between logic and mathematics

I’ve read the article in the SEP about the philosophy of mathematics. I believe I follow most of it. However, I am a bit puzzled by something that may be due to some basic misunderstanding on my part....
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What is Wittgenstein claiming when he says that “each thing can be the case or not the case while everything else remains the same”?

In Tractatus 1.21 Wittgenstein writes  Each item can be the case or not the case while everything else remains the same. [1] I'm looking for commentary on this point, since it seems out of place ...

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