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I How Often is First-Order and Modal Logic Used in Math?

  1. Dec 21, 2016 #1
    I'm a philosophy minor, who took first-order logic (philosophy department, not the math side) and I have a chance to take modal logic next semester (also under philosophy). It's going to be a much smaller class and will likely be harder. But it's also essential to doing work in and/or understanding some areas of philosophy (for example, philosophy of religion's ontological argument for God's existence is based on an argument in modal logic). In trying to gauge whether it would be worth my time, I'm wondering how often modal logic (and/or first-order) is used in math subjects?

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  3. Dec 21, 2016 #2


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    I have never come across any discipline in maths in which modal logic is useful, let alone necessary. I have yet to be convinced that there are any statements or deductions in modal logic that cannot be formulated in Classical First-Order Predicate Logic together with ZFC set theory.

    My understanding is that the main use of modal logic is to enable one to express concisely certain metaphysical hypotheses involving counterfactuals. They can be expressed without it, but the expression is less concise. The semantics of modal logic is also quite interesting. Since modal logic is the most-used lingo in certain philosophical disciplines, in particular the metaphysics of identity, free will and Kripkean philosophy of language, it is necessary to have at least a rudimentary understanding of it in order to make sense of readings in those areas.

    So, from a science and maths point of view, there is no benefit to learning modal logic. From a philosophy point of view, it depends what areas you want to focus on. In the more practical areas like ethics, politics and philosophy of life, it is not generally used.

    Having said that, modal logic is kind of fun so, if you don't have to give up on something useful in order to do it, why not?
  4. Dec 21, 2016 #3
    Well, there are people who specialize in mathematical logic, but I think they are sadly, increasingly rare, or kept as pets in computer science departments. :) Seriously though, there is some nice overlap in CS.

    Also as @andrewkirk pointed out, it's fun (for people like us) just on its own terms.

    -Dave K
  5. Dec 22, 2016 #4
    Yeah, that's really a bummer. I was hoping there was some transferability of those skills, because it's supposed to be a harder class. I'm not sure if I'll take it now - although, I'm still leaning towards it.

    I actually probably will and drop a different "hard" class mainly because I'm sick of not being able to read and analyze philosophy arguments using modal logic.

    Even the mind/soul-body argument has a modal structure.

    I totally agree that modal logic can be done for fun's sake. :biggrin: Honestly, that's one of the less practical reasons I'm considering it so highly!! I'm sure my parents would be thrilled that I'm using credits for personal enjoyment and curiosity. :-p
  6. Dec 22, 2016 #5
    That's cool. Too bad I'm not doing anything CS related (Econ. and History major with minors in Philosophy and Math).

    I agree that fun is always a factor and when I consider that I took a class on world music for an elective and will likely never use it, then I don't feel so bad taking modal logic. :)
  7. Dec 22, 2016 #6
    That actually sounds awesome. I have a friend who has a masters in Econ, is a history buff, and who is working on his PhD. in math right now. I stay out of arguments with him because I always lose.

    Well, here are 90 pages about the history of mathematical modal logic. So at least one major combined with two of your minors. :)

    -Dave K
  8. Dec 22, 2016 #7
    Of course studying ANY kind of logic or math helps your reasoning abilities.

    -Dave K
  9. Dec 22, 2016 #8


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    I would have signed this without hesitation, but then I've found this on Wikipedia (by chance and on a not quite serious search for "topological logic": "In abstract algebra and mathematical logic, topological Boolean algebra is one of the many names that have been used for an interior algebra in the literature."

    I'm sure modal logic isn't necessary to deal with Boolean algebras, so this remark is meant to be an interesting addition rather than a point of criticism.
  10. Dec 22, 2016 #9


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    I double majored in philosophy in undergrad, with a focus on logic. Modern modal logic is every bit as rigorous as any other first order predicate logic you'll run across. If it's taught from that perspective (rigor, as opposed to a historical perspective), it can actually be extremely valuable to further mathematical studies. Kripke semantics provides insight into model theory, completeness and incompleteness, etc., plus my classes got into issues that foundational mathematics classes don't normally get into. This includes technical things like intensionality versus extensionality and how that affects the identity relation, but also things with a distinctly philosophical flavor--think Lewis's metaphysics of modal realism. But at least the way I was taught, my modal logic classes had much more of a mathematical feel than a philosophical one.
    Caveat: I'm American, and the recent Anglo-American philosophical tradition is heavily influenced by the logical positivists (like Russell and early Wittgenstein), and so tends to appear a lot more mathematical than the trends in Continental European philosophy, which is heavier on the existentialists (like Sartre and Heidegger).
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