Why does the Principle of Explosion not make Mathematical Logic inconsistent?

-3

3

In Reductio ad Absurdum (RAA), we determine that a proposition P is false when it derives a contradiction. If we use this same derived contradiction as the premises to the Principle of Explosion (POE), we now prove this same proposition P is true. How does that not make Mathematical Logic inconsistent?

Reductio Ad Absurdum
(P → (A ∧ ¬A)) ⊢ ¬P

Principle of Explosion
(1) P → (A ∧ ¬A)
(2) (A ∧ ¬A) ⊢ P
(3) ∴ P

The above seems to show that we can correctly derive both P and ¬P thus making mathematical logic inconsistent.

Logical Inconsistency
two or more propositions are asserted that cannot both possibly be true.

polcott

Posted 2019-09-11T04:10:29.683

Reputation: 286

Comments are not for extended discussion; this conversation has been moved to chat.

– Geoffrey Thomas – 2019-09-13T08:44:49.117

No, logic will not work without disjunction elimination. It is part of 'introduction/removal harmony' which most folks would not discard. If I must turn left, then I must turn either right or left, and that is not negotiable to normal humans. Intuitionist reject his #3, because it leads to proofs where you get a result without a path. A combing of a hairy ball either does or does not exist. A ball with no such combing can't exist. Therefore every ball has such a combing -- but we can't construct it because it fell out of a rule, and was not the result of a reasoning process. – None – 2019-09-13T19:49:17.797

3Please stop editing the question further. If you have another question, completely changing one with some good answers already given is not the way we do things here. – Philip Klöcking – 2020-06-22T22:31:17.580

@PhilipKlöcking I think that I have it just about perfect. Improving my questions is my only chance to get my question asking restored. I – polcott – 2020-06-22T22:33:52.980

2That maybe the case, but you should also consider that there is a difference between perfectionalism and steadily shifting the goalposts of a question so that relevant, existing answers do not fully match the question anymore. This has also to do with respect towards thos who answered and this question here is certainly one you should not be worrying about as much as others. – Philip Klöcking – 2020-06-22T22:36:41.537

@PhilipKlöcking Try studying it. I think that I came up with something new. – polcott – 2020-06-22T22:36:51.423

@PhilipKlöcking I tried to keep that in mind when I made a slight shifts of focus. – polcott – 2020-06-22T22:37:53.770

5Coming up with something is not what questions are for. It just highlights that there is no genuine question, only presumption. – Philip Klöcking – 2020-06-22T22:37:57.900

@PhilipKlöcking At this point I could back out of most of the changes and provide my answer as an answer. I could also reedit my question so that it much more closely corresponds to a question and the existing answers. Does that sound good? – polcott – 2020-06-22T22:48:43.347

@PhilipKlöcking I rolled it back to a much earlier version that is consistent with the current answers. – polcott – 2020-06-23T05:19:10.783

Answers

-1

Principle of explosion In classical logic, intuitionistic logic and similar logical systems, the principle of explosion (Latin: ex falso [sequitur] quodlibet, 'from falsehood, anything [follows]'; or ex contradictione [sequitur] quodlibet, 'from contradiction, anything [follows]'), or the principle of Pseudo-Scotus, is the law according to which any statement can be proven from a contradiction.[1] That is, once a contradiction has been asserted, any proposition (including their negations) can be inferred from it; this is known as deductive explosion.

Logical Inconsistency
two or more propositions are asserted that cannot both possibly be true.

Principle of Explosion derives P
(1) (A ∧ ¬A) ⊢ P
(2) ∴ P

Principle of Explosion derives ¬P
(1) (A ∧ ¬A) ⊢ ¬P
(2) ∴ ¬P

Since the Principle of Explosion allows P ∧ ¬P to be derived from the same premise the Principle of Explosion makes logic inconsistent.

polcott

Posted 2019-09-11T04:10:29.683

Reputation: 286

16

Explosion is a property of logical consequence relations, and thus of logics, that is not trivial: Some logics have it, some don't. So there is simply no sense asking

Why can't we simply get rid of the Principle of Explosion?

If the logic you start with, say classical propostional logic, is explosive, then you cannot get rid of explosiveness and at the same time retain all properties of the intial logic. Rather what you do is change your logic. Whether that change is justified of course is determined by the field your logic is to be applied to. If your field of application is classical math, classical logic is fine and explosiveness a price worth paying, since it doesn't interfere with mathematical practice. On the other hand, if your logic is supposed to model data bases, where inconsistencies are likely to occur, explosiveness is no good. There you should choose some fragment of relevant logic like FDE.

That classical logic is explosive crucially depends on the following properties of its consequence relation:

  1. Simplification: A & B ⊢ A
  2. Disjunctive Weakening: A ⊢ A v B
  3. Disjunctive Syllogism: A v B, ~ A ⊢ B
  4. Cut rule: If M ⊢ A and N, A ⊢ B, then M, N ⊢ B

(Here M, N are sets of formulas and commas mean set union.) So, if you want to avoid explosion you must reject one of these properties. Most of the options have been played through in the literature. Parry's logic of analytic implication rejects (2), many relevant logics reject (3), Tennant's intuitionistic relevant logic rejects (4) so that logical implication is no longer transitive. But whatever option you choose, the resulting system is non-classical and that may also be a price to pay for some applications.

If you're interested in these matters of relevance you should read John Burgess's delightful piece 'No Requirement of Relevance', which appeared in the Oxford Handbook of Phil of Math and Logic.

sequitur

Posted 2019-09-11T04:10:29.683

Reputation: 1 348

I would get rid of (2). A = "I like ice cream" B = "Donald Trump ate an office building". I see no need for (2). – polcott – 2019-09-12T21:45:56.100

1

Then you're surely interested in Parry systems, i.e. logical systems, where logical consequence requires that premises and conclusion overlap in content. Thomas Ferguson recently published his dissertation on this subject. Chapter 4 particularly adresses the failure of disjunctive weakening in Parry systems.

https://pdfs.semanticscholar.org/7627/4e23147b000bbe23162b56dfd6773b263da4.pdf

– sequitur – 2019-09-13T15:17:57.533

It just seems to me that if there is no relevance connection between the premises and the conclusion that mathematical logic would be defined as a mere artifice that does not actually formalize the way that correct reasoning actually works. – polcott – 2019-09-13T15:20:41.013

@polcott But with the case of (2), the inference is "I know something, so therefore I know the thing i know and given that one or both of what i know and something else, must be true" isn't that intuitive? I'm not saying you shouldn't reject it, but it's not a surprising or counter-intuitive inference surely? – Daniel Prendergast – 2019-09-13T16:47:11.353

@DanielPrendergas If is allows BOE and is inessential then get rid of it to forbid BOE thus cause mathematical logic to more closely correspond to the way that human reasoning actually works. – polcott – 2019-09-13T17:09:07.117

The simplest logic that discards the principle of explosion is Mathematical Intuitionism. You lose the idea that not not A is always equivalent to A, and therefore the Law of the Excluded middle. In particular it can be contradictory for A not to exist and that does not imply that A exists in any usable sense, which makes a lot of math cumbersome. But the ways it becomes cumbersome have proved useful to consider deeply, in that they make math more like computer science, in useful ways. – None – 2019-09-13T18:27:25.063

2@jobermark Explosion does hold in intuitionistic logic. Maybe you're thinking of Reductio? – Eliran – 2019-09-13T19:22:05.037

2@EIiran It depends on how you look at it. A proof of something false does not prove an arbitrary fact. It holds officially in Heyting's formalism, but that ist not the standard of proof. Having a proof is the standard of proof. Any proof that actually used explosion would not be constructive. It is classically true that all elephants will be pink when pigs fly, but there is not a constructive proof leading from flying pigs to pink elephants. – None – 2019-09-13T19:33:12.400

8

Rather than give you the correct technical answer (edit: okay I ended up going into some technical details, oops), which has been provided, I think I'll try and diagnose where you're intuitions have led you astray.

It seems to me that you think that if a formal system can derive (P & ¬P), then we have to accept it and thus accept the resulting explosion. If I'm right, what you're doing is assuming that whenever we write a logical statement, we're endorsing it. But we're not, we're reasoning meta-logically. That is, to write "S ⊢ Q" is to say " suppose S ⊢ Q is a true statement in the logical system" not asserting it as true, and then trying to make it false later having already said it's true. If we were doing the latter, then yes, we'd have to accept the consequence of every logical statement we write.

But we don't have to, once we get the contradiction, accept the contradiction's truth. Instead, we can (actually, we have to) accept that whatever we used to prove the contradiction can't be true (nor derivable). What we've done is shown that some part of the set of statements IS NOT derivable because we assumed it was derivable and got contradiction. That is, what we've got is the following inference from our exercise:

Let S be a finite set of statements and Q be what we've derived from S. The question is, is S itself and instance of "⊢ S" or not? Suppose S ⊢ Q. But then suppose Q ⊢ (P & ¬P). You think that now we've got (P & ¬P), and given (P & ¬P) ⊢ A ( where A = any statement and its negation), we've now exploded the logic. But on the contrary, because assuming that "⊢ S" leads to S ⊢ (P & ¬P), we know that the assumption "⊢ S" must have been false. And because it's false, we know that the explosion "⊢ A" is also false. To accept (P & ¬P) as true would lead to explosion. But instead we've reasoned that, axiomatically, ⊢ ¬(P & ¬P), therefore anything deriving (P & ¬P) can't be derivable. Then we've shown that, were S derivable, (P & ¬P) would also be derivable. But this contradicts our axiom that ⊢ ¬(P & ¬P). From this it follows that S was never derivable in the first place. But if S is not derivable, then nor is anything of the form S ⊢ Q (Q standing for anything derived from S). But S ⊢ A is an instance of S ⊢ Q. So A is not derivable from S.

For the sake of completeness (in the non technical sense), lets also note that for every possible set of statements, if that set of statements derives A (still symbolising every possible statement, ie: explosion), the same proof above applies. Therefore, there is no possible set of statements that derive A. So not only do we know that a given set of statements S doesn't derive A, but that no set of statements derive A. Therefore "⊢ A" is ALWAYS false, and as such, this proves that explosion is impossible for our logical system. Isn't that a satisfying result we've now got ourselves to put our minds at rest?

When we write "⊢ S" in this (ludicrously informal) proof, we're not committed to it, nor any of its consequences. We just write it down as if it were the case, and as soon as we derive something impossible within the axiom schema and rules of inference our system is using from pretending its true, then we have a direct way of knowing it's false.

I hope I haven't left you in even more confusion.

Daniel Prendergast

Posted 2019-09-11T04:10:29.683

Reputation: 608

Why do we allow the principle of explosion to exist when we already know that contradiction ALWAYS means stop, do not proceed? – polcott – 2019-09-11T20:10:42.200

5@polcott The principle explosion exists for technical reasons of its own. It provides an argument for why we're right to reject contradictions, but it's not as if, once we derive a contradiction in our proof, we then continue to do the proof until we get explosion, and THEN say "look, we've got explosion; the contradiction is wrong". Explosion just follows from the contradiction irrespective of whether we've stop doing mathematical work having found a contradiction or not. SO, you derive contradiction, you derive explosion. Explosion is just a feature of contradiction. – Daniel Prendergast – 2019-09-11T20:39:55.860

See my edits for my reply. – polcott – 2019-09-11T20:41:55.317

3My comments still apply to your edit. You're right in that, once we have a contradiction, we know something is wrong with the reasoning we used to get that contradiction (which might have been our intention, or might be a mistake on our part). We don't, having found a contradiction, then prove the resultant explosion. Explosion is just a property that the logical system has if contradictions are true. In classical logic, explosion is always false because contradictions are always false. But in principle, we could define a system where contradictions are true and explosion results – Daniel Prendergast – 2019-09-11T21:03:47.227

4

Here is the question:

How is it that POE [the principle of explosion] does not make mathematical logic inconsistent?

If the axioms of mathematical logic were inconsistent then the principle of explosion would reduce mathematical logic to trivilism where all propositions are true since they can all be derived from that inconsistency.

However, having the principle of explosion does not make the axioms inconsistent. That would require a separate step. To show that the axioms are inconsistent one has to derive some proposition P and its negation ~P from those axioms (not from some other assumptions) prior to using that principle to derive all propositions.

Here is how Wikipedia represents that symbolically:

P, ¬PQ

The important thing to note is that we have to first derive both P and ~P. Then we can use explosion to derive Q.

Reductio ad absurdum is a way to derive the negation of an assumption should that assumption lead to a contradiction. Here is Wikipedia's description of it:

In logic, reductio ad absurdum (Latin for "reduction to absurdity"), also known as argumentum ad absurdum (Latin for "argument to absurdity"), apagogical arguments or the appeal to extremes, is a form of argument that attempts either to disprove a statement by showing it inevitably leads to a ridiculous, absurd, or impractical conclusion, or to prove one by showing that if it were not true, the result would be absurd or impossible.

That assumption is not one of the axioms. It is only tentatively assumed. That one can derive a contradiction from that assumption and other propositions is often what one desires to do so one can derive the negation of that assumption.


The OP adds this question:

Why is the Principle of Explosion (that starts with a contradiction) allowed when it is common knowledge that inference stops when reaching a contradiction?

One use of explosion in a natural deduction system is to handle one or both cases of a disjunction to derive a result using disjunction elimination (vE). Here is a proof showing how that works.

enter image description here

To eliminate the disjunction in line 1 and derive the goal P, I have to consider two cases: P and Q. The first case, P, is easy. It is already the goal I want to derive. The second case, Q, is more difficult. However, I also have an assumption on line 2, ¬Q, that I can use to derive a contradiction, symbolized by ⊥ on line 6. Now I use explosion on line 7 to derive P from that case as well. From explosion I could have derived anything, but what I need is P. So I derive that.

Having derived P in both cases I can use disjunction elimination to eliminate the disjunction on line 1, referencing both cases on lines 3-4 and lines 5-7 and derive P on line 8.

This would be one use of explosion in a natural deduction system.


Kevin Klement's JavaScript/PHP Fitch-style natural deduction proof editor and checker http://proofs.openlogicproject.org/

Wikipedia contributors. (2019, August 15). Reductio ad absurdum. In Wikipedia, The Free Encyclopedia. Retrieved 13:25, September 11, 2019, from https://en.wikipedia.org/w/index.php?title=Reductio_ad_absurdum&oldid=910921053

Wikipedia contributors. (2019, September 6). Principle of explosion. In Wikipedia, The Free Encyclopedia. Retrieved 13:21, September 11, 2019, from https://en.wikipedia.org/w/index.php?title=Principle_of_explosion&oldid=914233106

Frank Hubeny

Posted 2019-09-11T04:10:29.683

Reputation: 18 742

That does not really seem to answer my (now clarified) question. – polcott – 2019-09-11T16:40:43.283

@polcott In your edited question for RAA you are assuming: P → (A ∧ ¬A). That is not RAA. To use RAA, you are only allowed to assume P. From P and any other propositions, you have to actually derive A ∧ ¬A, not just assume you can. Only then can you derive ¬P. – Frank Hubeny – 2019-09-11T18:22:40.940

I could not find any concrete example of "proof by contradiction" that was simple enough, so I had to use the above propositional logic example (thus not concrete). Given (as in geometry) that P derives (A ∧ ¬A) then ¬P is proven. I really wish I could make this concrete and not merely have P derive a hypothetical contradiction. – polcott – 2019-09-11T18:28:18.197

@polcott If the axioms of mathematical logic are inconsistent then you would be able to derive A ∧ ¬A, but so far no one has been able to do so. However, if you assume that they are inconsistent, then you can derive every statement as true, that is, you get trivialism. But to show the axioms in fact are inconsistent more is needed than an assumption that they are. That contradictory result has to be derived. – Frank Hubeny – 2019-09-11T18:33:12.840

Do you have any simple concrete example of proof by contradiction that has English meanings attached to its propositional variables? I will give a terrible one so that my request is more clear: P = "All birds are yellow" I saw a green bird thus contradicting P, therefore P is false. – polcott – 2019-09-11T18:39:17.230

2@polcott The assumption you made that all birds are yellow and the statement that there exists a green bird would lead to a contradiction from which you can derive that all birds are not yellow. – Frank Hubeny – 2019-09-11T18:43:14.497

See the new question is my recent edits. – polcott – 2019-09-11T19:26:20.743

@polcott I updated my answer to address your new question. – Frank Hubeny – 2019-09-11T20:44:19.097

See my edits for my reply. – polcott – 2019-09-11T20:51:33.147

@polcott Given that the proof checker forces me to follow the inference rules, I have to use something like explosion in this case or change the rules. I do agree with you if the rules were different I may not need to so explicitly use explosion. – Frank Hubeny – 2019-09-11T20:55:30.740

This seems to be the first time that we agreed on anything. I would add to this that I don't think POE ever has any useful purpose. – polcott – 2019-09-11T21:00:42.737

@polcott It's not about having a "purpose". It's just a meta-mathematical property that some logical systems have, and others don't. An Explosive logic is one where, if that logical system allows contradictions to be true, every statement is provable. There are non-explosive logics; these are typically designed for the purpose of accepting true contradictions (Mostly to deal with the liar paradox by just saying "fuck it, lets allow it to be true"). A non explosive logic is one where, if you allow contradictions, not all statements are derivable. – Daniel Prendergast – 2019-09-11T21:16:45.577

@DanielPrendergas It really seems that the only actual correct way to handle contradiction is HALT ERROR DO NOT PROCEED. NO explosions, NO accepting contradictions as true. – polcott – 2019-09-11T21:51:15.813

@polcott But Explosion is a property of the system. When doing the proof, YOU, the logician, don't have to go and derive the explosion having found contradiction. But you're working in a logical system within which explosion results from contradiction. You don't have to do anything with that knowledge, no more than you'd have to use modus tollens if your proof of something relied entirely of modus ponens. If you want to get rid of contradiction, then you have to change disjunction elimination , and at that point you're just working in another system. – Daniel Prendergast – 2019-09-11T22:06:35.763

2@polcott Having a contradiction as an intermediate formula in a derivation does not amount to accepting it as true. Nor is there any reason to halt. And if it does appear as the final output, halting won't help, what can be derived by explosion can be derived without it. The theory is inconsistent and has to be replaced. So you are arguing for what everybody does anyway. – Conifold – 2019-09-12T11:33:06.123

@Conifold, so we could simply halt inference at every contradiction, getting rid of POE and inference would not change except that POE is eliminated? – polcott – 2019-09-12T14:49:15.547

@polcott Laws of logic do not come a la carte. You can make a personal commitment to never use POE, but it will still be there because it is a simple combination of disjunction introduction (if P then P or Q) and disjunctive syllogism (if P or Q, and not P, then Q) that even you use all the time. – Conifold – 2019-09-12T19:56:42.253

@Conifold POE is not the way that actual human reasoning actually works and on this basis it should be abolished. – polcott – 2019-09-12T20:15:53.087

@polcott The Earth rotating around the Sun is not the way that actual human observing actually works, and there is no abolishing it either. What matters is how things work, not how our brain evolved to process them over a series of random mutations. – Conifold – 2019-09-12T20:28:29.927

@Conifold Humans observe this through satellites. https://www.nasa.gov/mission_pages/soho/index.html

– polcott – 2019-09-12T20:32:31.973

2@polcott And they also reason through formal derivations, especially in math. – Conifold – 2019-09-12T20:33:39.530

3

Sequitur's answer is a good one, so I'm reluctant to add another, but he has addressed only some of the problems in the question, so here goes.

*1. There is no need to 'redefine' valid inference in terms of operations that are truth-preserving. Preserving truth is exactly what classical logic does. A classically valid argument guarantees that if the premises of an argument are true, then the conclusion is true. Expressed using the language of model theory this amounts to saying that every model of the premises is a model of the conclusion. Now bear in mind that in classical predicate logic 'all' does not imply 'some', so statements of the form "every A is a B" are true in the event that there are no A's. An inconsistent set of premises has no model, and therefore in such a case it is trivially true that every model of the premises is a model of the conclusion just because there aren't any. So any argument with inconsistent premises is classically valid.

*2. As Sequitur points out, there are many logics and some feature PoE and some do not. You are at liberty to choose to use some non-classical logic, such as one of the family of relevance logics. But there is a significant price to pay. Classical logic has many desirable features. It is provably sound and complete. It is Post-complete. Substantial fragments of it are algorithmically decidable. Its propositional calculus can be expressed using boolean algebra. It forms a satisfactory basis for first order arithmetic. There is a good reason why most logicians, and the great majority of mathematicians, stick to using classical logic.

*3. One cannot simply pick and choose between rules of inference within a logic. If you wish to dispense with a rule, such as disjunction introduction, it has consequences across all of the rest of the logic. Dropping the disjunction introduction rule changes the meaning of conjunction, negation and conditionals as well. In particular, it sacrifices bivalence: you can no longer rely on the principle that if a proposition is not true then it is false and if it is not false then it is true. Relevance logics have a much more complex semantics of negation.

*4. Disjunction introduction is not as weird as it first appears. For one thing, it is truth-preserving. Constructing a truth table for A v B will show that there is no row in which A is true and A v B false. Relevance logic is not truth-preserving: its natural semantics is typically expressed as something like preservation of information in a channel between sites or possible worlds. There is a section on the semantics of relevance logic in the SEP article.

*5. Although you have edited the question so that it no longer asks whether PoE makes logic inconsistent, you continue to include the 'proof'

(1) P → (A ∧ ¬A)
(2) (A ∧ ¬A) ⊢ P
(3) ∴ P

This is incorrect. The correct conclusion is P → P, which is a tautology and unproblematic.

*6. The PoE is actually useful in classical logic. An inconsistent theory explodes and contains all formulas as theorems. The contrapositive of this is that if there is a single formula that is not a theorem then the theory is consistent. This is used to prove the consistency of theories. It was used for example by Gentzen to prove the consistency of first-order arithmetic.

Bumble

Posted 2019-09-11T04:10:29.683

Reputation: 11 885

2

  • (1) P → (A ∧ ¬A)
  • (2) (A ∧ ¬A) ⊢ P
  • (3) ∴ P

No. The conclusion here should be P → P, via the Hypothetical Syllogism: P → Q, Q → R ∴ P → R .

Since P → P is a tautology there is no problem.

You might be more interested that (A ∧ ¬A) ⊢ P and (A ∧ ¬A) ⊢ ¬P are both valid.


Why is the Principle of Explosion (that starts with a contradiction) allowed when it is common knowledge that inference stops when reaching a contradiction?

Because it is not common knowledge that inference stops when reaching a contradiction. The very point of PoE is that when a contradiction is derived, anything may be inferred in that context -- including contradictions.

1. A ∧ ¬A   Premise
2. P        PoE 1
3. ¬P       PoE 1

Both these inferences are valid; which means each of the derived statements are considered at least as true as the statement from which they were inferred. Since that statement is a contradiction, P and ¬P are each guaranteed to be at least false.

Graham Kemp

Posted 2019-09-11T04:10:29.683

Reputation: 1 971

2

So the question is:

WILL LOGIC STILL WORK WITHOUT DISJUNCTION INTRODUCTION?

The answer is "work for what purpose?"

What I mean by this is that what you're trying to do here (more broadly), namely "get rid" of the principle of explosion is a traveled path (well in the latter half of the 20th century) broadly known as paraconsistent logics (Note the plural.) Dropping disjunction introduction is one (but not the only) way to achieve this goal--and it yields the so-called relevant (or relevance) logics. But as with most things in life, there are tradeoffs. (And if don't mind some historical backfitting, Aristotle's syllogistic logic is also paraconsistent, by construction, although quite limited in its proof theory.)

As for what could be "wrong" with relevance logic(s), even considering the most (and first) studied fragment that with implication, in the words of Anderson and Belnap (1975; p. 350) its proof theory is "of course not as easy as truth tables".

The semantics of relevance logics are a bit "weird" and with several proposals for "natural" candidates -- some more similar to models of modal logics. Ironically perhaps, understanding possible-world semantics of relevance logics is a bit more involved than even for modal logics, because one needs a [more obscure] notion of neighbourhood semantics for the former. On this angle, Mares' 2004 book is a gentle, self-contained introduction for philosophers. To quote a relevant (pun intended) bit from it:

It turns out that no finite valued truth table can capture relevant implication.

(Alas the proof is technical and you won't find it in Mares' book, but in Anderson and Belnap.)

A more serious problem is that [introducing] quantification in relevance logic is ahem... a real pain or rather that it has some unpleasant/unexpected consequences:

Unfortunately, Kit Fine (1988a) proved that the logic RQ (the logic R of relevant implication together with some standard quantificational axioms) is incomplete over the constant domain semantics.

If this seems too abstract of a problem to you, consider something more concrete: adapting the axioms of Peano Arithmetic (PA) to RQ is no biggie (all it takes is to replace "0 is not a successor" to "0 is not its own successor"). But the theory thus obtained (let's call it "relevant arithmetic") admits as model not only natural numbers, but also complex numbers! It also follows from this that some theorems provable in "classical" PA (stemming from the Chinese remainder theorem) are non-theorems in relevant arithmetic. (See Fridman & Meyer 1992 for proofs. In view of your related question, this latter aspect might perhaps convince you of one application/use of model theory as well.)


A further problem (see Mares, chapter 9) is that in order to have a sequent calculus with a deduction theorem, one needs to add an intensional conjunction (fusion -- &) and (not strictly necessary, but "looks nice") also an intentional disjunction (fission) on top of the "regular" (extensional) conjunction and disjunction of the relevance logic (R). In the absence of (at least one of) these additional operators in sequent calculus for R, the deduction theorem allows one to e.g. elevate the (extensional) disjunction into a deductive explosion. Fusion behaves somewhat differently than (extensional) conjunction; for example it's not idempotent, i.e. (A & A) -> A is not [a] valid [sequent] in relevance logic.(Unlike the Mares' book, the SEP treatment is a bit terse in this regard, as it only shows the Hilbert style systems e.g for R which does have one but not the other intensional operators in its axioms--one of them is technically redundant, as I said.)

Thus relevance logic plus its sequent calculus... starts to look a bit like some kind of linear logic (Although Mares doesn't make this connection in his book, the SEP treatment by the same author touches on the connection in its last paragraph though.)

Furthermore, it was shown by Read that one can take the intensional operators as a primitive and derive the (relevant, extensional) implication. (see sec. 9.6 "the Scottish plan" in Mares' book.) Also Read's own book is freely available nowadays and if/since you're interested in taking the intensional operators as primitives (given your motivation at the sequent calculus level), it may be a better read for you. See also the SEP entry on substructural logics which is also from this (sequent) perspective, where relevance logics are the sub-family that don't allow "weakening", which is what the latter article calls what you call "disjunction introduction".

Fizz

Posted 2019-09-11T04:10:29.683

Reputation: 516

"get rid" of the principle of explosion is a traveled path broadly known as paraconsistent logics<< Not at all. I am simply disallowing divergence from the basic model of the syllogism in that the major premise and the minor premise are semantically related to the conclusion. In the case of the syllogism this semantic relationship is established through set theory. – polcott – 2021-02-25T05:07:24.770

@polcott: Aristotle didn't do set theory. I suppose you mean naive set theory... – Fizz – 2021-02-25T06:00:35.120

If you want to get nit picky he did not do naive set theory either because this was not discovered until many centuries later. Within Aristotle's syllogism (logical necessity) prevents any principle of explosion result. https://en.wikipedia.org/wiki/Syllogism

– polcott – 2021-02-26T18:03:44.950

When we disallow divergence from the basic model of the Aristotle's categorical syllogism then a semantic connection between the premises and the conclusion is maintained on the basis of shared categories thus preventing any principle of explosion result: Major premise: All humans are mortal. Minor premise: All Greeks are humans. Conclusion: All Greeks are mortal. ∀ (H ∈ M) ∧ ∀ (G ∈ H) ∴ ∀ (G ∈ M) – polcott – 2021-02-26T20:43:42.157

-7

The POE only came to seem legitimate once mathematicians realised it was a necessary consequence of their system of logic at the time, essentially propositional logic based on the truth table of the material implication.

As they were unable to conceive of a better system, they choose to promote the POE to the dignified position of principle, effectively making mathematical logic contradictory to Aristotle's logic.

The POE is just one of a few mathematical logic's propositions that Aristotle would have dismissed as obviously false.

Successive generations of mathematicians just learn the stuff "at school" and most of them go on repeating all their lives, as if it was the Gospel truth, what they had to learn to get their exams.

This question is one of the most frequently asked question on the Internet, and yet I never saw anyone provide the beginning of a logical justification. All people do is provide irrelevant answers or simply reassert the principle as if repetition could be convincing. There is no possible justification beyond expediency.

Still, the POE is probably the best example of the limitation of human rationality, itself essentially based on deductive reasoning: Garbage in, garbage out. In this instance, start with the premise that the truth table of the material implication is the correct model of logical implication, and get the POE, and then some.

Speakpigeon

Posted 2019-09-11T04:10:29.683

Reputation: 2 400

We are exactly of one mind on this. – polcott – 2020-05-26T17:57:45.160

For some of the reasons that you just stated it seems that there is no correct translation of the following English sentence into predicate logic: "No dolphin sings unless it jumps." Every one of these translations produces a truth table with four rows for the two variables: of Sings and Jumps, thus inferring more than what was stated by the English

https://philosophy.stackexchange.com/questions/64298/how-to-translate-no-dolphin-sings-unless-it-jumps-into-predicate-logic/73171?noredirect=1#comment204891_73171

– polcott – 2020-05-26T18:06:42.603

Maybe you can provide this same sort of rationality to my discussion about foundationalism: https://chat.stackexchange.com/transcript/108022/2020/5/15

– polcott – 2020-06-03T04:51:08.167

The problem with "let's go back to Aristotle" is that you can't even (formally) do Euclidean geometry in Aristotle's logic. – Fizz – 2021-02-25T01:23:41.567

Have you tried any alterantive model of logical implication? they don't work. – user253751 – 2021-02-25T17:11:57.813