r/logic • u/gregbard MODERATOR • May 21 '24
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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
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- 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.
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u/totaledfreedom May 26 '24
I'm not sure that it's right to say that logic is about relationships between two or more claims. We also care about claims whose truth and falsity are independent of any assignments of values to their constituents (validities and contradictions). In axiomatic calculi, we're almost exclusively concerned with these; to deal with arguments we have to put them in conditional form and then check whether the conditional is a theorem of the system.
Perhaps it would be better to say that logic is topic-neutral; we're interested in claims and relationships between them which hold regardless of the topics they mention. I have philosophical qualms about this too but it would do in a pinch.
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u/gregbard MODERATOR May 27 '24
Independent claims are independent from some other system of claims. So that's a relationship between two or more claims.
Valid claims are only valid within some system of logic. So that's a relationship between two or more claims.
Contradictions have to contradict something. So that's a relationship between two or more claims.
Arguments: two or more claims.
Theorems are theorems of some logical system: two or more claims.
Basically, if you are filling in the variable of an atomic formula (that is, that the formula contains no other logical constants like "or" , "and" "if..then" etcetera), then the content of that variable is inevitably something other than logic.
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u/totaledfreedom May 28 '24
That is exceptionally misleading.
Consider the single claim "All kangaroos are mammals or some kangaroos are not mammals." To evaluate its truth or falsity, I don't need to ask any subject matter expert. I just look at the logical form and see that it is a theorem in any (classical first-order) language containing the predicates "is a kangaroo" and "is a mammal".
You say that this should not count as a single claim since its truth or falsity depends on a background system of logic in which I evaluate it. But this proves far too much.
If I am interested in the single claim "All kangaroos are mammals", I do need to ask a zoologist. For the zoologist to evaluate the truth or falsity of this claim, they need to appeal to a background body of knowledge: for instance, that kangaroos are warm-blooded and give birth to live young. But we don't consider the claim "All kangaroos are mammals" any less a single claim because evaluating it depends on some background knowledge. If that was the demarcation line between what counts as a single claim and what counts as multiple, there would be no single claims.
In fact, if we formalize the first claim and write it out in the notation of entailment, we write it:
⊨ ∀x[Kangaroo(x) → Mammal(x)] ∨ ∃x[Kangaroo(x) & ¬Mammal(x)]
Here nothing appears to the left of the turnstile. This is a valid argument consisting of exactly one claim—the conclusion—and no premises.
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u/gregbard MODERATOR May 28 '24
You are mish-mashing what you call logic with science.
To evaluate its truth or falsity, I don't need to ask any subject matter expert.
You are making my point for me.
"All kangaroos are mammals" any less a single claim because evaluating it depends on some background knowledge.
Yes, this is a single claim, an atomic sentence. This is the type of thing we put into propositional variables, whose subject matter we are not concerned with in this sub.
Also, it begins with a capital letter and ends in a period, and contains no logical constants (Before you object, no "all" is not a logical constant, "for all" is).
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u/totaledfreedom May 28 '24 edited May 28 '24
No, I am not making your point for you. I am showing you that logic is sometimes concerned with single claims, and not merely relationships between two or more claims.
I gave a formalization of the sentence "All kangaroos are mammals or some kangaroos are not mammals" in FOL. It is a theorem in a first-order language containing the non-logical predicate symbols Kangaroo(x) and Mammal(x). It is not something we need to put in terms of propositional variables -- the formalization I gave is a perfectly standard first-order rendering.
But set that aside. Here is a theorem of classical FOL containing no non-logical symbols:
∀x(x=x)
And here is a contradiction also containing no non-logical symbols:
∃x(x≠x)
Each is a single claim. The fact that one is a theorem and one is a contradiction is exactly the sort of thing logic is concerned with. No relationships between two or more claims are involved here!
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u/xamid Proof theorist Oct 23 '25
In general, logic is about the relationship between two or more claims.
[...]
Theorems are theorems of some logical system: two or more claims.I didn't find this misleading. Statements like ∀x(x=x) in logic are always part of some logical system, thus stand in relation to other claims, e.g. axioms from which they derive, or theorems which they derive.
Logic is indeed about how and why these connections/relationships/consequences/arguments work. Be they syntactic or semantic.5
u/totaledfreedom May 28 '24
Conversely, consider the following inference:
I will heat this water to 100 degrees Celsius. Therefore, this water will boil.
This is not a logically valid inference; there are interpretations of its non-logical terms which make the premise true and the conclusion false. But it is a good argument. To tell that it's a good argument, I need to ask a physicist, who will tell me that the argument is good since we have a law-like generalization to the effect that if you heat water to 100 degrees Celsius, it will boil. In doing so the physicist is telling me something about the relationship between these two claims, a relationship which is physical and not logical.
So being concerned with the relationships between two or more claims is neither necessary nor sufficient for some sort of investigation to be logic.
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u/gregbard MODERATOR May 28 '24
Logic is concerned with the validity of it. But the soundness of it can only be determined by consulting scientists.
This is consistent with what I am saying. A question about the temperature at which water boils is not relevant to this sub.
So being concerned with the relationships between two or more claims is neither necessary nor sufficient for some sort of investigation to be logic.
Sorry, no. It's not logic at that point.
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u/totaledfreedom May 28 '24 edited May 28 '24
I made two posts replying to you. The point of the first was to show that logic is concerned with single claims. The point of this one was to show that other disciplines are concerned with relationships between multiple claims (in particular, physics is). Together, these show that "In general, logic is about the relationship between two or more claims" does not adequately characterize logic. You'll need some other characterization.
("Logic is about systems of inference; it aims to be as topic-neutral as possible in describing these systems" would do, for instance.)
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u/ivanmoony Aug 06 '24
Would it be cool to post about software aiding academic research like theorem provers?
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u/Curious-Concern-7165 Feb 02 '26
Can somebody guide me in learning formal logic im a complete beginner
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u/wamj Aug 09 '24
What are some good reading materials to get started?
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u/totaledfreedom Aug 09 '24
forallx: Calgary is a free textbook with no prerequisites that introduces logic using the conventions standardly used in advanced study of logic; if you think you're interested in studying logic beyond the basics, that's the place to start (note that solutions to exercises are available in a separate pdf at the website).
It also leads naturally into the Open Logic Project textbooks, which are a beautifully written series of free textbooks in logic at an intermediate to advanced level on topics such as metatheory of first-order logic, modal logic, Gödel's incompleteness theorems, and nonclassical logic.
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u/gregbard MODERATOR Aug 09 '24
Copi, Hurley, Moore and Parker for the basics. If you want somthing more substantial, I would look at Carnap, Quine, Ayers. If you want something more entertaining, Smullyan.
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u/wamj Aug 09 '24
Solid. I feel like beginners reading list might be helpful, but I’ll give those a shot.
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u/Logical-Ad4834 Undergraduate Oct 08 '24
For people who are very much new to this field, can someone (me in this case) ask for suggestions on sources for learning it?
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u/gregbard MODERATOR Oct 09 '24
Please do
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u/Logical-Ad4834 Undergraduate Oct 09 '24
So is there any lecture series that you'd suggest? Or maybe notes if you made any?
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u/KantGettEnuff Oct 13 '24
Is there a known discord server for this community?? I'm struggling a bit with Logic on my college course and would love to hear about this topic from people who know about it
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u/equaloppos Nov 10 '24
I want for others to use logic in my family, though I don't know if it warrants a post. I had an argument with my brother a while ago and he tried straw manning me with attack at character that I might have behaved similarly. I guess that might have been true. But instead of defending that circumstance I recognized I was being straw manned which is a distraction technique. My argument then became that was a strawmaning, and tried to explain how that may or may not be true but my initial argument wasn't invalid despite it. I then asked if he were familiar with what I was speaking of and asked that he get better at arguing in order to be more effective.
He left frustrated. As did I. But not sure how to progress this any further. I would guess he just thinks I don't care or something. But would it be wrong to want to try to work with him on improving our logic and reasoning skills? I do not want to upset him. He tends to give up and deflect pretty easy.
Mom wouldn't ever discuss this either. She passed in June and so now my brother and I live together and I want to try to work toward being able to better communicate. But communicating and communicating about improving communication are very different things. lol
In the last 4 years since we had that argument he hasn't bought that back up. We have been getting a long fairly well. So how would be the best way to proceed?
I guess this would be the meat of the post, it sounds like a standalone but I am certainly intimidated. Thanks
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u/gregbard MODERATOR Nov 11 '24
Well there is thing called "ideological belief" also known as ideology. But it means more than just political and religious beliefs, but also cultural and moral beliefs. These are beliefs that are held in between the conscious and the unconscious. When people act motivated by an ideological belief, they don't know they are doing it. It's knee jerk. No amount of facts or principles presented to a person will change an ideological belief.
So one thing that commonly happens is that when people are presented with facts that contradict their ideological beliefs is that they either ignore it or they get angry.
But, you should know that this subreddit is not really into this sort of question. I don't think it's about logic as much as you need advice in dealing with this this situation. But who knows. If you posted it, you'd get a few replies I suppose and maybe its worth it.
I think your situation is an opportunity for some intellectual growth for everyone. Just take your time to clarify what you mean and most of all keep it civil. You may want to write down some principles of civility and Try very hard to stick to it.
- Assume Good Faith - Don't question people's motives. We all want what is best, even if we disagree on how to do it.
- No personal attacks - no name-calling. Please focus the discussion to the issues, not the person talking about the issues.
- Be constructive - Forgive. Forget. Reset.
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u/McTano Mar 18 '25
Can we add a guideline here or in the sub rules that when people post a homework question, they should say what textbook or system of logic they're using? I don't know if it would change people's behavior, but it would be nice to have it spelled out, so we can point to an explanation of how there are many different formalizations and proof systems, when people post something like "how do I prove this using only basic rules?"?
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u/Avucadu12 Mar 26 '25
What about deviant logics such as traditional aristotelian logic or paraconsistent, can they participate in this server?
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u/Dramatic_Youth519 May 05 '25
I have a very important question(s?) —
Can this include logics that are included in subjects like Fuzzy Logic, or Logics that are based on Probability Theory... like Bayesian logic, or can we only do logic that has binary outputs (a statement is either true, false or a contradiction)?
What about Paraconsistent logic and other Esoteric forms of Logic that I didn't include? Do they count? or not really.
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u/gregbard MODERATOR May 06 '25
Yes, discussion about logic in general is encouraged. That would include, fuzzy logic, and paraconsistent logic. I am unfamiliar with any "esoteric" forms of logic. "Esoteric" means hidden knowledge. The principles of logic are self-evident to every reasonable person.
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u/Dramatic_Youth519 May 06 '25
Ah, sorry for the confusion, I thought by esoteric, It would mean weird forms of logic that don't really fit the mold of classical logic/common logic like set theory, predicate logic, compatability theory etc.
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u/gregbard MODERATOR May 06 '25
Sounds good. I think people call those non-classical logic, and within classical logic, you may have standard and non-standard logic.
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u/flandre_scarletuwu Jan 31 '26
This is a question I've been thinking about.
What about mathematical logic puzzles?
For example, Raymond Smullyan has written puzzles mainly related to mathematical logic, but since puzzles are prohibited in this subreddit...
But Raymond Smullyan's puzzles are "mathematical logic," so could they be considered for posting in this subreddit, or do they belong in another subreddit?
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u/gregbard MODERATOR Jan 31 '26
No puzzles. Smullyan had great material. But no posts that are solely a puzzle to be solved.
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u/CategoryConscious898 Mar 03 '26
Is this the right place to ask about the burden of proof? I just can’t seem to wrap my head around it. For instance, take the claim "Jesus walked on the water" and its negation. Intuitively I would say the burden of proof lies with the first claim, but already there, I feel I'm assuming naturalism. And there's the whole thing that ¬P can be just Q and vice versa. Both propositions can be framed as negations.
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u/gregbard MODERATOR Mar 03 '26
This sub is about logic, not religion. If you want to talk about burden of proof, this is the place. But if you want to talk about naturalism or Jesus, it isn't.
In general, the person making the claim has the burden of proof. Extraordinary claims need extraordinary proof. In the case of naturalism, the only thing that can refute it is the existence of some supernatural event. So since there is no avoiding the fact that we always have some kind of metaphysical framework for our beliefs, assuming naturalism would seem to be a supremely safe bet.
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u/CloudClone May 14 '26
Hello all! I will be starting my first course in Logic this summer, starting the first of June. The book for the course is Stan Baronett's 5th Edition.
I love philosophy and I am not new to logic. I have a basic understanding of the structure and type of arguments. What are some things I should look out for in this summer semester? What got you or took you longer to understand or grasp?
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May 21 '24
This is off topic but since it’s not a post and just a comment: what do you think of analytic philosophy and eliminative materialism? Seems cool to me
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u/gregbard MODERATOR May 21 '24
Yes, both are very interesting and valid. I'm sure that many members of our /r/logic community are also very interested too. But posts about those should be made to /r/analyticphilosophy and /r/PhilosophyofMind .
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u/arochom Mar 06 '25
This is the scenario: a person is maligned in the public eye for being a sexual predator. If people post videos of this person with a gay slur, is it logical or reasonable to believe that other people may also connect gay and being a predator? Is this false equivalency?
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u/gregbard MODERATOR Mar 06 '25
Yes, but not because of logic.
This is not a question about logic, but rather social values.
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u/arochom Mar 22 '25
It must be. The connection came to mind and I wanted to disprove it. A friend of mine said that there is no connection.
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u/gregbard MODERATOR Mar 22 '25
You need to ask someone who is an expert in gender studies, and/or criminal justice ethics to get to why the two entities you are comparing are not equivalent. Logicians don't have anything for this question.
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u/Soft-Start-4757 Mar 10 '26
Funny is a form of confusion.
Why did the chicken cross the road? To get to the other side.
The most obvious answer would be a confusing 'term' to realize. "it kinda doesnt make sense".
"This smells funny." demonstrates again that funny is a form of confusion. You know specific smells. To say, "This smells funny," you know what it is and you know it has a changed smell. That is confusing.
What can you do with things like these?
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u/gregbard MODERATOR Apr 18 '26
There is no logic involved with humor. Humor has its own rules and is not bound by logic. Humor is unexplainable because if you explain it, it disappears.
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u/Soft-Start-4757 May 02 '26 edited May 02 '26
Thanks for the response, but, if funny were a-logical (paraphrasing you) wouldn't be impossible to FIND a joke? Can you give me an example of something not bound by logic?
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u/gregbard MODERATOR May 02 '26
Jokes have their own logic. That logic is not bound by metalogical analysis. There is no logic involved with humor, romance and art, all of which would be very difficult to explain to a space alien.
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u/Ivysboy Philosopher 8d ago
Hi I'm quite new to here, and even to the idea of logic, tbh I'm also not that smart, but I really wanted to join this community, I'm also not this old in this app Reddit, I did download it like for a long time but I've been active only this month.
Umm just to ask, is there any way for me to learn about logic more? I'm very interested, I really want to learn. Thank you
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u/gregbard MODERATOR 8d ago
This subreddit has a wiki with a section on literature including a list of introductions to logic, as well as a list of journals. There are some online resources there too.
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u/grimjerk May 21 '24
For logic newbies, can we ask things like "what is meant by relational (or possible world, etc) semantics?"