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. I always thought that the principle of explosion is ridiculous.

The answer to this question seems to be that we can't ever derive the kind of contradiction that POE needs from RAA so inconsistency is never derived.


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

It seems to me that a contradiction in inference is essentially the same thing as dividing by zero in computation. This makes it seem quite foolish to proceed onward with inference to explosion. Why is POE not abolished?

In the (Frank Hubeny) example of disjunction elimination there was no actual need for POE
(1) (P v Q)
(2) ¬Q
(3) ∴ P

Why can't we simply get rid of the Principle of Explosion?
The updated rule would be that inference halts at after contradiction in exactly the same way that computation halts when divide by zero occurs. POE is simply not the way that human reasoning really works. In courtroom testimony a contradiction indicates perjury.

@DanielPrendergast says:
"Well, you'll have to get rid of one of the following: Distjunction introduction A ⊢ A V B"

Yes that one seems to be a good one to get rid of.
A="I like ice cream", B="Donald Trump ate an office building".

The answer to this question seems to be that getting rid of POE requires us to get rid of disjunction introduction which seems like an utterly useless appendage.


  • Comments are not for extended discussion; this conversation has been moved to chat. – Geoffrey Thomas Sep 13 '19 at 8:44
  • 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. – user9166 Sep 13 '19 at 19:49
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    Please 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 Jun 22 at 22:31
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    That 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 Jun 22 at 22:36
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    Coming up with something is not what questions are for. It just highlights that there is no genuine question, only presumption. – Philip Klöcking Jun 22 at 22:37

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.

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  • We are exactly of one mind on this. – polcott May 26 at 17:57
  • 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 philosophy.stackexchange.com/questions/64298/… – polcott May 26 at 18:06
  • Maybe you can provide this same sort of rationality to my discussion about foundationalism: chat.stackexchange.com/transcript/108022/2020/5/15 – polcott Jun 3 at 4:51

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.

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  • 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 Sep 12 '19 at 21:45
  • 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. pdfs.semanticscholar.org/7627/… – sequitur Sep 13 '19 at 15:17
  • 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 Sep 13 '19 at 15:20
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    @jobermark Explosion does hold in intuitionistic logic. Maybe you're thinking of Reductio? – Eliran Sep 13 '19 at 19:22
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    @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. – user9166 Sep 13 '19 at 19:33

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.

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  • Why do we allow the principle of explosion to exist when we already know that contradiction ALWAYS means stop, do not proceed? – polcott Sep 11 '19 at 20:10
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    @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 Sep 11 '19 at 20:39
  • See my edits for my reply. – polcott Sep 11 '19 at 20:41
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    My 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 Sep 11 '19 at 21:03

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

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  • That does not really seem to answer my (now clarified) question. – polcott Sep 11 '19 at 16:40
  • @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 Sep 11 '19 at 18:22
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    @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 Sep 11 '19 at 18:43
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    @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 Sep 12 '19 at 11:33
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    @polcott And they also reason through formal derivations, especially in math. – Conifold Sep 12 '19 at 20:33
  • (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.

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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.

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