Law of excluded middle

topic posted Mon, March 12, 2007 - 2:45 PM by  VoodooChild
Aristotle coined the law of excluded middle (LEM). It's usually expressed in modern propositional logic as (P v ~P), P or not-P, using "or" in the exclusive sense. Something cannot both be P and not P (whatever quality P refers to). I think this also implies that something has to be either P or not P, it can't be neither.

I was going to ask whether people think this rule is "valid" but it occurs to me that "valid" usually implies "valid within some system," and that's not quite what I'm trying to get at. Many thinkers (Alfred Korzybski, Hans Reichenbach, Jan Lukasiewicz, Robert Anton Wilson) have developed "non-Aristotelian systems" that deny the law of excluded middle or some other basic Aristotelian assumption. How about a discussion of the general "reasonableness" of LEM? Does this help our thinking or muddle it?

My hunch is that LEM probably confuses us more than it helps, but I'd like to hear other peoples' thoughts on this.
posted by:
VoodooChild
SF Bay Area
  • Re: Law of excluded middle

    Mon, March 12, 2007 - 2:54 PM
    • Re: Law of excluded middle

      Tue, March 13, 2007 - 9:51 AM
      To make things more confusing, let's throw in the "tetralemma" of Buddhist logic:

      P is true
      P is false
      P is both true and false
      P is neither true nor false

      My understanding of this is hazy but I think the above 4 statements are supposed to understood simultaneously. That is, whatever statement P represents is supposed to be in some sense true, in some sense false, in some sense both true and false, and in some other sense neither true nor false. This is not a system of logic (in the robust sense of having rules for derivation and the like) but more like a way of approaching conceptual muddles, and turning simplistic ideas of truth and falsity on their heads (and perhaps mocking language itself as inadequate to experience?). If anyone has a better understanding of this, help me out here.
      • Re: Law of excluded middle

        Sat, March 17, 2007 - 11:31 AM
        The locus classicus for the tetralemma in Buddhist philosophy is Nagarjuna's Mulamadhyamakakarika. While a strong formulation of the tetralemma does not in fact appear in the work, it's clearly what he's driving at.

        My view is that if you look at how Nagarjuna argues in the MMK, it''s clear he must have held to the law of the excluded middle. His argument is reductio ad absurdum (Prasangika). Nagarjuna sets out to refute the concept of inherent existence. To do so he establishes that if you try to claim that phenomena inherently exist you will be forced to defend contradictory and absurd statements. If there were no law of the excluded middle, this would not be a problem - one would feel free to contradict one's self.

        Countless illustrations of his reliance on the law of the excluded middle can be found in his work. I openned MMK to a random page and immediately found:

        "An existent and nonexistent agent
        Does not perform an existent and nonexistent action.
        Existence and nonexistence cannot pertain to the same thing.
        For how could they exist together? "
        - MMK VIII, 7

        It's clear that Nagarjuna at least regarded the law of the excluded middle as a core principle of reason. And every formulation of the tetralemma in Buddhist philosophy is ultimately based on Nagarjuna. I regard him as one of the two or three most important philosophers in world history, along with Aristotle and perhaps Kant.

        The question then becomes, what does the tetralemma mean, given that he accepts the law of the excluded middle? For a thorough answer to this question I would refer you to Tom Tillemans's outstanding essay "Is Buddhist Logic Non-classical or Deviant?". His conclusion, which I quite agree with, is that the tetralemma does not mean:

        P is true
        P is false
        P is both true and false
        P is neither true nor false


        What it actually means is:

        X does not exist such that P is true
        X does not exist such that P is false
        X does not exist such that P is both true and false
        X does not exist such that P is neither true nor false

        This is a classical (i.e. non-contradictory) logical argument. In this case "X" is "a phenomenon that exists inherently", i.e. without depending on anything else. The tetralemma expresses the general form of his argument - that if one asserts inherently-existent objects, one will be forced to defend contradictory conclusions.
        • Re: Law of excluded middle

          Sat, March 17, 2007 - 11:59 AM
          I'd add as a historical note that by the seventh century CE, Buddhist logicians in India had derrived as complex and nuanced a system of formal logic as has existed in the world. Particularly the logico-epistemologists Dignaga and Dharmakirti.
          • Re: Law of excluded middle

            Wed, March 21, 2007 - 6:02 AM
            >> I'd add as a historical note that by the seventh century CE, Buddhist logicians in India had derrived as complex and nuanced a system of formal logic as has existed in the world. Particularly the logico-epistemologists Dignaga and Dharmakirti. <<

            If you could recommend any books on Buddhist Logic and/or on or by Nagarjuna (specific editions/translations) that would be greatly appreciated by me.

            As far as LEM goes I tend to see it as more a tool for cutting through false dichotomies than anything else. I don't think that it is universally applicable - but when someone (including a voice in your head) starts to promote an "either or" point of view, then such a claim intrinsically applies LEM to itself. If the proposed dichotomy violates LEM then the original idea should be discarded - or possibly reworked.
            • Re: Law of excluded middle

              Fri, March 30, 2007 - 2:34 PM
              Curt, I'm interested in your idea that LEM is a tool for cutting through false dichotomies, because my concern is that I think LEM creates them. Could you say more about what you mean?
        • Re: Law of excluded middle

          Mon, March 19, 2007 - 11:14 AM
          Sweet, thanks Barnaby! I'll have to think on this.
          • Re: Law of excluded middle

            Tue, March 20, 2007 - 3:40 PM
            My main problem with LEM is that it asks us to think in binary - to classify any given proposition as true or false - when many propositions seem resistant binary classification. I fear that this might encourage and justify rigid black and white thinking, and that worse yet, it might encourage people to justify their binary thinking by appeal to "The laws of logic." In my experience, people move rather quickly from "X is a
            rule of formal logic system Y" to "X is an eternal law of correct thought."

            Here are a few examples of cases where following LEM might mislead us:

            A proposition like "I am in the kitchen" seems like something that would easily yield a simple truth value, but imagine I am standing in the doorway between the kitchen and the dining room, with one foot in the kitchen and the rest of me out. Is the proposition "I am in the kitchen" true or false in this case?

            How might we classify the proposition "A can of Budweiser is white"? LEM seems to tell us we must classify this as either true or false, but both answers would be misleading.

            How about the paradox "This sentence is false" What is its truth value?

            One more for the road: "colorless green dreams sleep furiously" This sentence is famously nonsense, but binary logic with its LEM does not have a "nonsense" category.
            • Re: Law of excluded middle

              Thu, March 22, 2007 - 12:01 AM
              Hi Curt,

              > If you could recommend any books on Buddhist Logic and/or on or by Nagarjuna (specific editions/translations) that would be greatly appreciated by me.

              There's actually a good translation of Nagarjuna's magnum opus on line, here:
              www.stephenbatchelor.org/verses2.htm

              If you're looking for a book, Jay Garfield's "Fundamental Wisdom of the Middle Way" is a good translation of that same work, and it includes a long commentary. I don't agree with a lot of the commentary, but it's a good can openner.

              If you have a background in western philosophy, you might like "The Emptiness of Emptiness", which is a study of another classic of the philosophy of emptiness, the Madhyamakavatara, that was written by Nagarjuna's follower, Chandrakirti.

              I'm not sure what to recommend for an introduction to Buddhist logic - some of the work out there is badly outdated, and a lot of it is highly specialized. Foundations of Dharmakirti's Philosophy by John Dunne might be good, but I'm not sure how readable it'll be for the beginner.

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