r/logic • • May 21 '24

Meta Please read if you are new, and before posting

67 Upvotes

We encourage that all posters check the subreddit rules before posting.

If you are new to this group, or are here on a spontaneous basis with a particular question, please do read these guidelines so that the community can properly respond to or otherwise direct your posts.

This group is about the scholarly and academic study of logic. That includes philosophical and mathematical logic. But it does not include many things that may popularly be believed to be "logic." In general, logic is about the relationship between two or more claims. Those claims could be propositions, sentences, or formulas in a formal language. If you only have one claim, then you need to approach the scholars and experts in whatever art or science is responsible for that subject matter, not logicians.

"Logic is about systems of inference; it aims to be as topic-neutral as possible in describing these systems" - totaledfreedom

The subject area interests of this subreddit include:

  • Informal logic
  • Term Logic
  • Critical thinking
  • Propositional logic
  • Predicate logic
  • Non-classical logic
  • Set theory
  • Proof theory
  • Model theory
  • Computability theory
  • Modal logic
  • Metalogic
  • Philosophy of logic
  • Paradoxes
  • History of logic
  • Literature on Logic

The subject area interests of this subreddit do not include:

  • Recreational mathematics and puzzles may depend on the concepts of logic, but the prevailing view among the community here that they are not interested in recreational pursuits. That would include many popular memes. Try posting over at /r/mathpuzzles or /r/CasualMath .

  • Statistics may be a form of reasoning, but it is sufficiently separate from the purview of logic that you should make posts either to /r/askmath or /r/statistics

  • Logic in electrical circuits Unless you can formulate your post in terms of the formal language of logic and leave out the practical effects of arranging physical components please use /r/electronic_circuits , /r/LogicCircuits , /r/Electronics, or /r/AskElectronics

  • Metaphysics Every once in a while a post seeks to find the ultimate fundamental truths and logic is at the heart of their thesis or question. Logic isn't metaphysics. Please post over at /r/metaphysics if it is valid and scholarly. Post to /r/esotericism or /r/occultism , if it is not.


r/logic • • Jul 06 '26

Meta Free Online Logic Resources

24 Upvotes

The r/logic wiki now includes free online resources to learn logic (courses, books, and proof tools).

If you know of any others, please provide links so they can be added in future.


r/logic • • 16h ago

Philosophy of logic What normative criteria can justify the choice between different logics in concrete philosophical argumentation, and are those criteria sufficient to ensure that the choice is not arbitrary?

Thumbnail
8 Upvotes

r/logic • • 12h ago

Propositional logic My first substack article

Thumbnail
crazykazoo.substack.com
5 Upvotes

Hello, this is my first substack blog post. It is kind of short and weirdly written, but I still would appreciate any thoughts.

Edit: the article's title is: "If AND is like Multiplication and (X)OR is like Addition Then What is Division?" And it is basically about finding an operator or function that would be analogical to division but for propositional logic (supposing AND is multiplication)


r/logic • • 19h ago

Philosophical logic Program in Formal Philosophy

0 Upvotes

Hi. I'm 13 and I study analytic philosophy. I'm building a multiparameter method for evaluating philosophical schools: Φ⁺ for macro-philosophy (ontology, modality, fundamentality) and Φ⁻ for micro-philosophy (epistemology, ethics). A prototype of Φ⁺ already exists (LaTeX document, working formula, pilot on Platonism/nominalism/structuralism).

I'm looking for:

Logicians — to formalize consistency conditions for Φ⁺ and Φ⁻.

Philosophers — to independently score 3–5 schools (inter-rater reliability).

Programmers — to build a small calculator for weights and sensitivity analysis.

Critics — to stress-test the method with counterexamples.

Systematizers — to organize parameters, scales, and documentation.

This is not "philosophy is fully formalizable." It's "evaluation can be structured" if we separate levels, declare normative assumptions, and calibrate on classic schools.

If interested — DM me. Age 13–22, but what matters is willingness to argue and check.


r/logic • • 1d ago

Philosophy of logic The Illusion of Meaning: Is Math Just Playing with Dashes?

Post image
1 Upvotes

​

Episode 1

Let's start with three completely meaningless symbols: p, q, and - (a dash). In Chapter 2 of Gödel, Escher, Bach, Hofstadter introduces a formal system called the pq-system, which does nothing but blindly shuffle these characters based on a few basic typographical rules. Imagine a machine completely devoid of understanding, just churning out strings like --p---q-----.

This is exactly where the magic of human cognition—and the core concept of isomorphism—kicks in. Our brains detect a pattern. If we map p to "plus," q to "equals," and treat the dashes as numbers, that sterile string suddenly snaps into a universal truth: 2+3=5. We project form and meaning onto lifeless symbols.

Hofstadter uses this to demonstrate that meaning isn't an intrinsic property of symbols. It emerges from discovering the structural correspondence between formal systems and reality. Meanwhile, the system's rules couldn't care less about our interpretations; the mechanism grinds on exactly the same way whether we see it as profound mathematics or just a tally of apples and oranges.

What are your thoughts on this? Don't these striking isomorphisms suggest that math actually resides in its own independent, abstract realm, making our formal systems nothing more than the nets we weave to catch those pre-existing truths?

— If_P_Then_Panic


r/logic • • 2d ago

Philosophy of logic Frege, can we say A=A is mandatory a priori?

15 Upvotes

A=A or "Hesperus is Hesperus” simply mean the same thing; therefore, it is analytical and a priori.

But if we say,

"(This) Odyssey is (the) Odyssey."

Can we count that as a priori too? Or is it both analytical and synthetic?


r/logic • • 2d ago

Paradoxes Special pleading is a fallacy except in the case of this sentence.

3 Upvotes

It was intended jokingly, but how would you analyze the sentence: "Special pleading is a fallacy except in the case of this sentence"?


r/logic • • 2d ago

Propositional logic Need help understanding Polish Notation

5 Upvotes

Hello, I’ve recently started studying mathematical logic on my own and came across a problem I had with a textbook I was reading (introduction to mathematical logic Mendelson second edition) the excersize in question asked me to write (C v ((B ^ (~D)) ⊃ C )) in Polish notation. I came up with the answer vC ^ B ~D ⊃ C.
However according to the book the answer is v C ⊃ ^ B ~DC. I’m very confused how it could come to that answer and the book itself is pretty old so I haven’t been able to google a proper answer. Would someone be willing to explain to me both how I got it wrong and the steps that it takes to annotate the problem properly? Thank you in advance.


r/logic • • 2d ago

Philosophy of logic Theological question about limits of human use of logic

0 Upvotes

If we claim that logic has limits when speaking about God, what logical grounds justify that claim?

TLDR: On empirical grounds we can reasonably justify that human use of logic is restricted and has limits when talking about Allah, Blessed Be He and may He bless and greet His Final Messenger Muhammad ﷺ.


r/logic • • 2d ago

Computability theory Questions on the asymptotic limits of an algorithmic state-space damping heuristic

0 Upvotes

I am analyzing a parameterized dynamic scaling function designed to model state-space transitions. The heuristic function is formulated as f(n) = 2^(n / ((p(n))^2 * (3n+1))), where p(n) is a strictly increasing prime-product (primorial n#) and (3n+1) is a linear dynamics operator. Asymptotically, as n approaches infinity, the exponent collapses to 0, forcing f(n) to achieve absolute convergence to 1.0 at a super-exponential decay rate of O(e^(-2n)). I am trying to formalize its implications on a Nondeterministic Turing Machine (NDTM) configuration tree under a proposed Dynamic Branching Law, B(n) = 2^(f(n) - 1), where the compression drives the effective branching factor to exactly 1.0 in the limit. I would highly appreciate structural feedback from the logic community on these points: 1. Is the first-order asymptotic decay bound of O(e^(-2n)) derived via Taylor expansion robust enough to mathematically bound non-deterministic expansion? 2. Does an asymptotic compression of a branching factor to 1.0 formally imply a structural reduction of the search space, or is it isolated as a non-constructive mathematical artifact? 3. Which intermediate automata-theoretic or resource-bounded information frameworks are best suited to formalize these structural bottlenecks? Thank you for your time and guidance!


r/logic • • 3d ago

Paradoxes Is there a hardest question?

16 Upvotes

Suppose a hardest logically solvable question H.

Asking if H is the hardest question could then become H

But then we can repeat the process again, since "Asking if H (original) is the hardest question" is now our new H we can just do the same to it, then the same to the new one and over and over again.


r/logic • • 2d ago

Philosophy of logic Math Has Limits We Can Never Cross: Some truths are real even if we can never prove them.

Thumbnail
thoughtframe.org
0 Upvotes

r/logic • • 4d ago

Literature "On the Hierarchy of Natural Theories" by James Walsh. Excellent survey on some modern research in mathematical logic

Thumbnail cambridge.org
34 Upvotes

r/logic • • 3d ago

Academic Community ANU Logic Summer School 2026 – Looking for Male Roommates in Canberra

Thumbnail
2 Upvotes

r/logic • • 3d ago

Philosophy of logic From Logical Positivism to Formal Heuristics: What did the death of verificationism teach us about mathematical ontology?

Thumbnail
1 Upvotes

r/logic • • 4d ago

Proof theory How to solve this

Post image
8 Upvotes

Thanks!


r/logic • • 5d ago

Propositional logic How to prove one conditional from another?

6 Upvotes

I'm doing some practice exercises in natural deduction, but the exercises don't come with an answer key, so I really have no clue how to figure this out:

A → (B → C). Therefore (A ∧ B) → C

I get as far as assuming A in a subproof, assuming B in a further subproof, and connecting A and B via conjunction introduction, but then I have no clue how to get back to showing that (A ∧ B) → C. What am I missing here? My thinking is that I need to prove that A and prove that B, so that if I assume their conjunction I can show that their conjunction implies C. But I have no idea how to do this in a way that allows me to cite the subproofs.

Any help is appreciated!


r/logic • • 4d ago

Philosophy of logic Why do we still treat Mathematics as "Ultimate Truth"? A critique of mathematical platonism and axiological bias.

Thumbnail
0 Upvotes

r/logic • • 5d ago

Paradoxes Why 'Heap' Is a Word That Breaks Logic: Remove one grain of sand from a heap. Is it still a heap? Keep going.

Thumbnail
thoughtframe.org
0 Upvotes

r/logic • • 7d ago

Set theory I wanted to see how a fully formalized theorem looks like. And I need to clear my understanding of proof structure. I have attached ZFC axioms and First order logic gentzen sequent calculus. So is it such that the proof trees of theorems have ZFC axioms in Gamma here?

Thumbnail
gallery
83 Upvotes

additionally most undergrad math theorems like lagrange's subgroup can be written in this form?
Could I find atleast one theorem presented in such format?
And where all could the group axioms go? gamma or leafs?


r/logic • • 7d ago

History of logic How War Changed the Way We Use Logic: A soldier wrote a masterpiece while facing death in the Great War.

Thumbnail
thoughtframe.org
10 Upvotes

r/logic • • 7d ago

Logical fallacies Is there a fallacy for this?

Thumbnail
0 Upvotes

If somone said this "Cant believe you wrote a dissertation on my one paragraph", in order to undermine my point or get a laugh from thier audience or to not put in effort to combat my point.

'Dissertation' would be very much hyperbolic.

Would i be gish galloping? I believe my points were succint to the best of my ability.


r/logic • • 9d ago

Question Question from my law school admission exam in Romania. Do you know any similar exams in your country? Im looking for more practice tests to solve

Post image
10 Upvotes

r/logic • • 9d ago

Literature Relearning logic from scratch, so I made a free book for myself (AI-drafted, needs real eyes on it)

8 Upvotes

Hi everyone,

I have a B.S. in Computer Science and really enjoyed the discrete mathematics course in my first three semesters. But I started working as a software engineer early, my focus drifted to IT-specific topics, and I never made my logic knowledge solid. Now I want to relearn it properly from scratch.

So I put together a self-study book for myself, drafted with an AI assistant, and I’m sharing it in case it helps someone else:
https://swicherwich.github.io/cogito-ergo-sum/

What’s in it: a “from absolute zero” part (reasoning, math language, sets and functions, a symbol atlas), propositional logic and natural deduction, predicate logic with careful treatment of the ∀/∃ rules, proof technique and induction, an overview of soundness, completeness and Gödel’s theorems, and a small Python proof checker and resolution prover with tests. It’s free (CC BY-SA) and works on phones, with dark mode and a dyslexia-friendly font.

Honest caveats: it hasn’t been reviewed by a logician, so there are probably mistakes, and the metatheory chapters are condensed rather than fully rigorous. If you spot an error or an unclear explanation, please open an issue: https://github.com/swicherwich/cogito-ergo-sum/issues

Again, hope it help somebody, and thank you for the feedback!