Destructive dilemma[1][2] is the name of a valid rule of inference of propositional logic. It is the inference that, if P implies Q and R implies S and either Q is false or S is false, then either P or R must be false. In sum, if two conditionals are true, but one of their consequents is false, then one of their antecedents has to be false. Destructive dilemma is the disjunctive version of modus tollens. The disjunctive version of modus ponens is the constructive dilemma. The destructive dilemma rule can be stated:

Destructive dilemma
TypeRule of inference
FieldPropositional calculus
StatementIf implies and implies and either is false or is false, then either or must be false.
Symbolic statement

where the rule is that wherever instances of "", "", and "" appear on lines of a proof, "" can be placed on a subsequent line.

Formal notation

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The destructive dilemma rule may be written in sequent notation:

 

where   is a metalogical symbol meaning that   is a syntactic consequence of  ,  , and   in some logical system;

and expressed as a truth-functional tautology or theorem of propositional logic:

 

where  ,  ,   and   are propositions expressed in some formal system.

Natural language example

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If it rains, we will stay inside.
If it is sunny, we will go for a walk.
Either we will not stay inside, or we will not go for a walk, or both.
Therefore, either it will not rain, or it will not be sunny, or both.

Proof

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Step Proposition Derivation
1   Given
2   Given
3   Given
4   Transposition (1)
5   Transposition (2)
6   Conjunction introduction (4,5)
7   Constructive dilemma (6,3)

Example proof

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The validity of this argument structure can be shown by using both conditional proof (CP) and reductio ad absurdum (RAA) in the following way:

1.   (CP assumption)
2.   (1: simplification)
3.   (2: simplification)
4.   (2: simplification)
5.   (1: simplification)
6.   (RAA assumption)
7.   (6: De Morgan's Law)
8.   (7: simplification)
9.   (7: simplification)
10.   (8: double negation)
11.   (9: double negation)
12.   (3,10: modus ponens)
13.   (4,11: modus ponens)
14.   (12: double negation)
15.   (5, 14: disjunctive syllogism)
16.   (13,15: conjunction)
17.   (6-16: RAA)
18.   (1-17: CP)

References

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  1. ^ Hurley, Patrick. A Concise Introduction to Logic With Ilrn Printed Access Card. Wadsworth Pub Co, 2008. Page 361
  2. ^ Moore and Parker

Bibliography

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  • Howard-Snyder, Frances; Howard-Snyder, Daniel; Wasserman, Ryan. The Power of Logic (4th ed.). McGraw-Hill, 2009, ISBN 978-0-07-340737-1, p. 414.
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