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Lately I've been reading about Quentin Meillassoux, and it seems that the only law of logic he doesn't see as contingent is the law of non-contradiction, because if the world is what it is not, then there cannot be absolute contingency, and contingency is absolute. Charles Williams Johns explains it like this:

no contradiction can exist or else it would be both what it is and what it is not and hence has no room for real contingency and subsequently becomes necessary (i.e. it cannot be destroyed if an entity is both what it is and what it is not simultaneously)

I do not, however, understand how he could not therefore accept the law of identity as absolute. If A cannot not equal A, then does that not mean that A=A necessarily? Could this somehow be fixed by the law of the excluded middle being contingent?

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    The law of non-contradiction is completely meaningless unless you have several other axioms and inference rules. Negation doesn't mean anything without introduction and elimination rules for negation, for instance. You can't do any deduction if all you have is non-contradiction.
    – causative
    Apr 12 at 20:07
  • People, on this site as in most places, just conflate Logic proper with mathematical and/or formal logic: the one foundational category of Logic proper indeed is the notion of non-self-contradiction. Se P.F. Strawson, Introduction to Logical Theory, for an excellent/classic introduction to Logic proper. Apr 12 at 21:51
  • Yes, standard FOL has no "=" symbol, and hence no "law of identity". Identity is a metaphysical notion and is not part of logic proper. Non-contradiction has no need for "=" , it is just ¬(P ∧ ¬P). Even if you include "=" symbol there is no contradiction in ¬(A=A), so non-contradiction does not get you A=A, even with LEM.
    – Conifold
    Apr 12 at 22:41
  • @Conifold "Logic proper" has nothing to do with ~(P/\~P) either... Just please do not attach to the labels I introduced the very meaning I have said they haven't. :) Apr 12 at 22:55
  • So what is the meaning then? Your post says nothing about it. How do we get from impossibility of P(W) and ¬P(W) to W = W? You seem to be interpreting "the world is what it is not" as ¬(W = W), but that is not what it says.
    – Conifold
    Apr 12 at 23:04

3 Answers 3

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Can the law of non-contradiction exist without the law of identity?

in what follows, we'll tackle the 0th-order law of identity, the scheme 'p → p' for some binary connective for which modus ponens holds (here we're taking modus ponens to be the defining property/characteristic of any 'implication-like' connective): the the 2nd-order cases involving '=' can then be analysed via leibniz, and the 1st-order cases may be dealt with in appropriate theories, so the crux of the matter seems to be the 0th-order case

Could this somehow be fixed by the law of the excluded middle being contingent?

consider intuitionistic propositional logic, and define a binary connective @ as

p @ q := ¬p ∨ q

then MP holds, that is

p, p @ q ⊢ q

so @ is, at least in this (weak) sense, an implication, but of course

p @ p := ¬p v p

does not necessarily hold

this may be considered 'cheating' somewhat as ¬ is in principle defined in terms of the usual intuitionistic implication...

a certain variation on the previous example provides an example in which excluded middle still holds:

consider some (normal) modal logic in which at least axiom T/reflexivity holds, and that 'p → □p' does not in general hold, define

p @ q := ¬p ∨ □q

and check that MP holds (using T), plus also LNC and LEM, but that 'p @ p' does not necessarily so

you also may find interesting stuff regarding "implication problems" in dalla chiara and giuntini's chapter on "quantum logics" in gabbay's handbook of philosophical logic

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In the way the terms LNC and identity are standardly used within logic, they are independent. Classical logic includes both, but you could have a non-classical logic that lacks either one or lacks both.

If someone wishes to talk about contingency, then it is important to say what kind. Contingency, like possibility, comes in many varieties. There is logical possibility, physical possibility, epistemic possibility, deontic possibility, metaphysical possibility, actual possibility, and no doubt many others. Ditto for contingency and for absoluteness.

Much confusion follows from not being specific. In fact, the two pieces of text that you quoted sound like claptrap to me. Maybe if I knew the context they would be comprehensible. What is "contingency is absolute" supposed to mean? What does a "contradiction... cannot be destroyed" supposed to mean? Contradictions are propositions; they are not things that might or might not exist in the world but features of how we describe the world and reason about it. In classical logic all contradictions are false; that doesn't mean they "don't exist".

The text you quote also seems confused about whether it is talking about propositions, things, or beliefs. Usually LNC is understood to be about propositions: it states that ¬(P ^ ¬P) is a logical truth. This should not be confused with talk about things. It is different to say that a thing never both exists and does not exist. It is also different from saying that it is irrational both to believe and disbelieve the same proposition at the same time.

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  • Meillassoux views contingency as necessary, and all other necessity as nonexistent or at least conditional. As far as I can tell, Meillassoux basically accepts LNC as the only law of logic which necessarily describes what happens in reality (he rejects the principle of sufficient reason). I don't get the difference between there not being able to be contradictions in the world and the inability to have true contradictory propositions.
    – edelex
    Apr 13 at 22:34
  • A contradiction is a proposition that is always false. The world doesn't contain propositions; it contains things. (Or, at least, that is a fairly commonsense way of approaching it.) We don't examine the world and discover it to be contradictory. But we can formulate contradictory propositions. However odd the world may be, we can always find some way of describing it that doesn't require us to say that some contradiction is true. (Speaking classically here: there are some logicians who hold that some contradictions are true.)
    – Bumble
    Apr 13 at 23:48
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Can the law of noncontradiction exist without the law of identity?

I am going to say no. Here is a summary of the three laws of thought, from the online Encyclopedia Britannica (formatting altered):

—————

[T]raditionally, the three fundamental laws of logic [are]: (1) the law of contradiction, (2) the law of excluded middle (or third), and (3) the principle of identity.

The three laws can be stated… as follows.

(1) For all propositions p, it is impossible for both p and not p to be true[.]

(2) Either p or ∼p must be true, there being no third or middle true proposition between them[.]

(3) [A] thing is identical with itself, or (∀x) (x = x), in which ∀ means “for every”; or simply that x is x.

—————

There are different ways to play with these ideas. In the absence of the assumption that something is what it is, then something (p) can be anything at all; p can still be itself, but it does not have to be. In short, p can be not-p. This conclusion does indeed destroy the law of noncontradiction, for it becomes possible for p and not-p to be true simultaneously.

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  • does this not assume that the law of identity is talking about propositions being identical to themselves? I thought it was about things themselves
    – edelex
    Apr 13 at 22:43
  • @edelex. I was assuming that the law of identity is talking about both propositions and things. What do you mean? Apr 14 at 18:03

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