Category
Logic & argument
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What’s the difference between a valid argument and a sound one?
In a debate thread here someone said an argument was "valid but not sound" and I realised, being completely new to logic, that I didn't understand the difference. To me both words just sounded like "good". Could someone explain simply, with examples…
Started by Kavya Menon3 replies -
How does one contradiction let you prove absolutely anything?
A friend studying computer science told me that in logic "a contradiction implies everything". If you accept one contradiction, you can prove the moon is green, that I'm a king, anything. This sounds like a trick. Contradictions are bad, sure, but why…
Started by Fajar Nugroho3 replies -
What do the strange names Barbara, Celarent and Darii actually mean?
I'm working through an old logic textbook my uncle left at our house. There is a page with a Latin verse that starts "Barbara, Celarent, Darii, Ferioque..." and then a lot of other names like Cesare and Baroco. The book just says…
Started by Fajar Nugroho4 replies -
Why does logic count ‘if the moon is cheese, then 2 + 2 = 5’ as true?
Half my jokes are conditionals. So I was pleased to find a whole branch of logic about "if". Then I hit the truth table and it broke my brain. Apparently in propositional logic "if P then Q" is false only when P…
Started by Gemma Pritchard4 replies -
Can someone explain what the Nyāya logicians meant by vyāpti (pervasion)?
My training is entirely in Western formal logic, and I've started reading Matilal's The Character of Logic in India. I understand the standard example: there is fire on the hill because there is smoke, and wherever there is smoke there is fire,…
Started by Tomasz Szewczak4 replies -
What is “defeasible” reasoning, and why do people say most good arguments aren’t valid?
I read philosophy on the bus home. Last week I read that "most good real-world reasoning is not valid at all", and the word that kept coming up was defeasible. Here is my problem. In the kitchen I reason all day. The…
Started by Joel D'Souza4 replies -
Every invoice needs some approver: why does the order of quantifiers change the meaning?
One of my clients, a small business, has an internal policy that says: "Every invoice must be approved by some manager." Two of their managers have now spent a month arguing about it. One reads it as: each invoice needs a manager's…
Started by Harpreet Kaur Grewal4 replies -
What is pragma-dialectics, and what makes a “critical discussion” different from ordinary arguing?
In a moot court workshop a visiting lecturer kept referring to "pragma-dialectics" and "the rules for a critical discussion", as though everyone knew what they were. I nodded and didn't. From what I've gathered, it comes from Frans van Eemeren and Rob…
Started by Deepa Kartha4 replies -
How can I tell an argument is valid when I don’t know what its words mean?
Forgive me if this is basic; I'm very new to formal philosophy. I met this pair of arguments in a logic primer: All whales are mammals. All mammals are warm-blooded. So all whales are warm-blooded. All zorps are blickets. All blickets are…
Started by Kavya Menon4 replies -
Could a culture reason by different logical rules, or is that a colonial myth?
Every year a student in my lycée class asks me some version of this: "Is logic universal, or is it just how Europeans think?" The history makes the question sharp. In the early twentieth century, Lucien Lévy-Bruhl described so-called primitive peoples as…
Started by Khady Sène5 replies -
Frege and Husserl started on the same side. What was the psychologism dispute about?
I've been reading about the split between analytic and continental philosophy because my colleagues treat it as two tribes. What surprised me is that both traditions are said to trace back, around 1900, to two men who agreed about something: Frege and…
Started by Pieter van Dijkhuizen5 replies -
Outside a courtroom, who should carry the burden of proof?
In court the burden of proof is fixed by rules. In a criminal case the prosecution must prove guilt beyond reasonable doubt; in a civil case the claimant must win on the balance of probabilities. Islamic law has a well-known maxim to…
Started by Hamza Qureshi6 replies -
Soundness and completeness of a logical system: what exactly is being proved?
I'm stuck on vocabulary, having no formal training in logic. I've read that a logical system can be sound and complete: everything provable is true, and everything true is provable. Fine. But then I read that Gödel proved a completeness theorem in…
Started by Pieter van Dijkhuizen4 replies -
If syllogistic logic couldn’t handle relations, why did it last two thousand years?
Working inside predicate logic every day, I'm often reminded how much it can do that the syllogism can't. The textbook example (usually credited to De Morgan) is: all horses are animals, therefore every head of a horse is the head of an…
Started by Derek Hollis6 replies