# Principle of explosion

The principle of explosion (Latin: ex falso quodlibet, "from a falsehood, anything follows", or ex contradictione sequitur quodlibet, "from a contradiction, anything follows"), or the principle of Pseudo-Scotus, is the law of classical logic, intuitionistic logic and similar logical systems, according to which any statement can be proven from a contradiction.[1] That is, once a contradiction has been asserted, any proposition (or its negation) can be inferred from it.

As a demonstration of the principle, consider two contradictory statements – “All lemons are yellow” and "Not all lemons are yellow", and suppose (for the sake of argument) that both are simultaneously true. If that is the case, anything can be proven, e.g. "Santa Claus exists", by using the following argument:

1. We know that "All lemons are yellow" as it is defined to be true.
2. Therefore the statement that (“All lemons are yellow" OR "Santa Claus exists”) must also be true, since the first part is true.
3. However, if "Not all lemons are yellow" (and this is also defined to be true), Santa Claus must exist – otherwise statement 2 would be false. It has thus been "proven" that Santa Claus exists. The same could be applied to any assertion, including the statement "Santa Claus does not exist".

The principle is not a universal rule; rather it exists as a consequence of a choice of which logic to use. It does not appear in some paraconsistent logics which allow localised 'gluts' of contradictory statements to be proved without affecting other proofs. In artificial intelligence and models of human reasoning it is common for such logics to be used. This can also occur in formal science, for example quantum mechanics and relativity lead to contradictions in extreme environments, but these contradictions do not imply that Santa exists – only that there are some scenarios where one or both theories are in need of alteration. Truth maintenance systems are AI models which try to capture this process.

## Symbolic representation

The principle of explosion can be expressed in the following way (where "$\vdash$" symbolizes the relation of logical consequence and "$\bot$" symbolizes a contradiction) :

$\{ \phi , \lnot \phi \} \vdash \psi$
or
$\bot \to P$.

This can be read as, "If one claims something ($\phi\,$) and its negation ($\lnot \phi$), one can logically derive any conclusion ($\psi$)."

## Arguments for explosion

An informal, descriptive, argument is given above. In more formal terms, there are two kinds of argument for the principle of explosion, semantic and proof-theoretic.

### The semantic argument

The first argument is semantic or model-theoretic in nature. A sentence $\psi$ is a semantic consequence of a set of sentences $\Gamma$ only if every model of $\Gamma$ is a model of $\psi$. But there is no model of the contradictory set $\{\phi , \lnot \phi \}$. A fortiori, there is no model of $\{\phi , \lnot \phi \}$ that is not a model of $\psi$. Thus, vacuously, every model of $\{\phi , \lnot \phi \}$ is a model of $\psi$. Thus $\psi$ is a semantic consequence of $\{\phi , \lnot \phi \}$.

### The proof-theoretic argument

The second type of argument is proof-theoretic in nature. Consider the following derivations:

1. $\phi \wedge \neg \phi\,$
assumption
2. $\phi\,$
from (1) by conjunction elimination
3. $\neg \phi\,$
from (1) by conjunction elimination
4. $\phi \vee \psi\,$
from (2) by disjunction introduction
5. $\psi\,$
from (3) and (4) by disjunctive syllogism
6. $(\phi \wedge \neg \phi) \to \psi$
from (5) by conditional proof (discharging assumption 1)

This is just the symbolic version of the informal argument given above, with $\phi$ standing for "all lemons are yellow" and $\psi$ standing for "Santa Claus exists". From "all lemons are yellow and not all lemons are yellow" (1), we infer "all lemons are yellow" (2) and "not all lemons are yellow" (3); from "all lemons are yellow" (2), we infer "all lemons are yellow or Santa Claus exists" (4); and from "not all lemons are yellow" (3) and "all lemons are yellow or Santa Claus exists" (4), we infer "Santa Claus exists" (5). Hence, if all lemons are yellow and not all lemons are yellow, then Santa Claus exists.

Or:

1. $\phi \wedge \neg \phi\,$
hypothesis
2. $\phi\,$
from (1) by conjunction elimination
3. $\neg \phi\,$
from (1) by conjunction elimination
4. $\neg \psi\,$
hypothesis
5. $\phi\,$
reiteration of (2)
6. $\neg \psi \to \phi$
from (4) to (5) by deduction theorem
7. $( \neg \phi \to \neg \neg \psi)$
from (6) by contraposition
8. $\neg \neg \psi$
from (3) and (7) by modus ponens
9. $\psi\,$
from (8) by double negation elimination
10. $(\phi \wedge \neg \phi) \to \psi$
from (1) to (9) by deduction theorem

Or:

1. $\phi \wedge \neg \phi\,$
assumption
2. $\neg \psi\,$
assumption
3. $\phi\,$
from (1) by conjunction elimination
4. $\neg \phi\,$
from (1) by conjunction elimination
5. $\neg \neg \psi\,$
from (3) and (4) by reductio ad absurdum (discharging assumption 2)
6. $\psi\,$
from (5) by double negation elimination
7. $(\phi \wedge \neg \phi) \to \psi$
from (6) by conditional proof (discharging assumption 1)

Paraconsistent logics have been developed that allow for sub-contrary forming operators. Model-theoretic paraconsistent logicians often deny the assumption that there can be no model of $\{\phi , \lnot \phi \}$ and devise semantical systems in which there are such models. Alternatively, they reject the idea that propositions can be classified as true or false. Proof-theoretic paraconsistent logics usually deny the validity of one of the steps necessary for deriving an explosion, typically including disjunctive syllogism, disjunction introduction, and reductio ad absurdum.
The metamathematical value of the principle of explosion is that for any logical system where this principle holds, any derived theory which proves $\bot$ (or an equivalent form, $\phi \land \lnot \phi$) is worthless because all its statements would become theorems, making it impossible to distinguish truth from falsehood. That is to say, the principle of explosion is an argument for the law of non-contradiction in classical logic, because without it all truth statements become meaningless.