Is it possible to translate a syllogism into propositional logic? This is the example: All doctors went to medical school. Hanna is a doctor. Hanna went to medical school. Thanks a lot, Sebastiano

For any syllogism containing quantifiers such as "all," "some," and "no"/"none," you'll need predicate logic for the translation. Propositional logic alone won't suffice. But you could use propositional logic to translate a non-quantified argument that's at least similar to the syllogism: "If Hanna is a doctor, then she went to medical school. Hanna is a doctor. Therefore, Hanna went to medical school."

Is there any single genuinely correct logic or so called all-purpose logic? If not, why should we find it?

I presume that you would dismiss out of hand the following answer to your first question: "Yes, there is a single genuinely correct, all-purpose logic, and there is no such logic, and there is more than one such logic." So I take it that your question presupposes that no correct logic could allow that answer to be true. If you're asking whether there's any good reason to abandon the standard, two-valued, "classical" logic routinely taught to university students in favor of some non-classical logic, then I'd answer no . Some philosophers say that we ought to adopt a non-classical logic in response to such things as the Liar paradox or the Sorites paradox, but their arguments for that conclusion have never struck me as persuasive. I think that the Liar and the Sorites can be solved using only classical logic (and bivalent semantics), or at least it's too early to conclude that they can't be. For a much more detailed answer, you might consult Susan Haack's book Deviant Logic, Fuzzy Logic:...

Recently I asked a question about logic, and the answer directed me to an SEP entry, which then took me to two other SEP entries, on Russell's paradox and on the Liar's paradox. Frankly, after having read through those explanations, there was a glaring omission from every cited philosopher, and I wondered if everyone was overcomplicating things: I don't see how there is any "paradox" at all. Consider the concept of a "round square" or a "six-sided pentagon." Those are nonsensical terms, because of the structural nature of the underlying grammar. They are neither logical nor illogical, they are merely grammatically inconsistent at the fundamental level of linguistic definition. The so-called "paradox" of Russell and the Liar seem to me to be exactly the same kind of nonsensical formulations: the so-called "paradox" is merely a feature of the language, these concepts also are grammatically inconsistent at the fundamental level of linguistic definition. Russell's "paradox" is just as "paradoxical" as...

If I may, I think you're being a bit too dismissive of Russell's paradox. We start with the observation that some sets aren't members of themselves: the set of stars in the Milky Way galaxy isn't itself a star in the Milky Way galaxy; the set of regular polyhedra isn't itself a regular polyhedron; and so on. It seems that we've easily found two items that answer to the well-defined predicate S: is a set that isn't a member of itself . Naively, we might assume that a set exists for every well-defined predicate. (For some of those predicates, it will be the empty set.) But what about the set corresponding to the predicate S? This question doesn't seem, on the face of things, to be nonsensical or ungrammatical. But the question shows that our naive assumption implies a contradiction, and therefore our naive assumption can't possibly be true.

What is the difference between "either A is true or A is false" and "either A is true or ~A is true?" I have an intuitive sense that they are two very different statements but I am having a hard time putting why they are different into words. Thank you.

I presume that you're using the formula "~ A" to abbreviate "It is not true that A" rather than "It is false that A." If my presumption is wrong, then this response may not answer your question. Where A is some proposition , I see no difference between "It is not true that A" and "It is false that A": Every proposition that isn't true is false, and every proposition that isn't false is true. However, the same doesn't hold if A is, instead, some sentence . For a sentence can fail to be true without being false. To use an admittedly controversial example : the self-referential sentence "This sentence is not true" is neither true nor false, because the sentence fails to express any proposition in the first place (including the proposition that the sentence isn't true!). Any false sentence is not true, but a sentence can fail to be true without being false. But perhaps you meant to use the formula "~ A" to represent rejection or denial of the sentence or proposition A. Some philosophers...

If there could be a counter-argument against a premise, does that make the premise false and the argument unsound?

No. The mere possibility of a counter-argument (i.e., "there could be a counter-argument") doesn't imply that the premise is false or that an argument containing the premise is unsound. The counter-argument itself must have a true conclusion in order to guarantee that the premise against which it's a counter-argument is false. Every sound argument has a true conclusion (although the converse doesn't hold), so if there exists a sound argument against a particular premise, then the particular premise is false. Often, however, the very soundness of that counter-argument will be a matter of controversy.

I'm having a difficult time determining if a certain math problem should be classified as using Formal or Informal Logic. Here it is: 1. ALL except 2 of my pets are dogs. 2. ALL except 2 of my pets are cats. 3. ALL except 2 of my pets are birds. Q: How many pets do I own? A: 2 or 3 So, while it's obvious why the answer could be 3, it's not obvious how it could be 2 as well. The reason why is because the phrase "All" could be zero, which would represent an empty set. And, of course, I could own pets other than the ones mentioned (fish / lizards). So, knowing that, we can substitute that example back into the original problem as follows: I own two, pets, which are both fish. All except 2 of my pets are dogs, which in this case, is equal to zero. So, the set of dogs can possibly be an empty set. So, anyways, I was wanting to know if the puzzle itself could be considered "formal", or is it informal because most people would mean "All" to at least equal one, and we add that assumption in there?

I interpret you as asking this: Why do we find it puzzling or counterintuitive that statements 1–3 are true in the case in which you own exactly two pets, neither of which is a dog, a cat, or a bird? Is it because we assume that "all" implies "at least one"? Those are empirical, psychological questions whose answers I don't know. But I do think it's worth distinguishing between what "all" logically implies and what "all" conversationally implies. (You might have a look at the SEP entry on implicature .) On the one hand, the statement "All intelligent extraterrestrials are extraterrestrials" had better be true, and its truth had better not depend on the existence of intelligent extraterrestrials. So I think there's good reason to deny that "all" logically implies "at least one." On the other hand, someone who owns no dogs and who says "All my dogs have their shots" has said something odd or misleading, even if true. So I think there's good reason to say that, at least sometimes, "all"...

On April 10, 2014, in response to a question, Stephen Maitzen wrote: "I can't see how there could be any law more fundamental than the law of non-contradiction (LNC)." I thought that there were entire logical systems developed in which the law of non-contradiction was assumed not to be valid, and it also seems like "real life" suggests that the law of non-contradiction does not necessarily apply to physical systems. Perhaps I am not understanding the law correctly? Is it that at most one of these statements is true? Either "P is true" or "P is not true"? or is it that at most one of theses statements is true? Either "P is true" or "~P is true"? In physics, if you take filters that polarize light, and place two at right angles to each other, no light gets through. Yet if you take a third filter at a 45 degree angle to the first two, and insert it between the two existing filters, then some light gets through. Based on this experiment, it seems like the law of non-contradiction cannot be true in...

Because the present questioner refers to my reply to Question 5536 , I'll chime in here to clarify what I said there. My point was about the fundamentality of LNC. I wrote, "I can't see how there could be any law more fundamental than the law of non-contradiction (LNC)." I gave the following reason: "Let F be any such law. If the claim 'F is more fundamental than LNC' is meaningful (and it may not be), then it conflicts with the claim 'F isn't more fundamental than LNC' -- but that reasoning, of course, depends on LNC. " So that's why no law could be more fundamental than LNC, because LNC would need to be true before (in the sense of logical priority) the claim that some other law is more fundamental would even make sense. If someone can make sense of the claim that some law is more fundamental than LNC, I'm all ears.

What's the difference between understanding an opponent's argument, and agreeing with it? What prevents me from saying that if my opponent disagrees with my argument, he must misunderstand it?

Nothing prevents you from saying that, but then nothing prevents you from being wrong when you say it. If your argument is deductive, you might make progress by asking your opponent which (if any) premise in your argument he/she finds implausible and which (if any) inference in your argument he/she finds invalid. If your opponent rejects your conclusion, try finding out why he/she doesn't regard your argument as persuasive support for your conclusion.