Constructive dilemma

From Wikipedia, the free encyclopedia

Transformation rules
Propositional calculus
Rules of inference Modus ponens Modus tollens Biconditional introduction Biconditional elimination Conjunction introduction Simplification Disjunction introduction Disjunction elimination Disjunctive syllogism Hypothetical syllogism Constructive dilemma Destructive dilemma Absorption Rules of replacement Associativity Commutativity Distributivity Double negation De Morgan's laws Transposition Material implication Material equivalence Exportation Tautology
Predicate logic
Universal generalization Universal instantiation Existential generalization Existential instantiation
v t e

Constructive dilemma^[1]^[2]^[3] is a 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 P or R is true, then Q or S has to be true. In sum, if two conditionals are true and at least one of their antecedents is, then at least one of their consequents must be too. Constructive dilemma is the disjunctive version of modus ponens, whereas, destructive dilemma is the disjunctive version of modus tollens. The rule can be stated:

${\frac {P\to Q,R\to S,P\lor R}{\therefore Q\lor S}}$

where the rule is that whenever instances of " $P\to Q$ ", " $R\to S$ ", and " $P\lor R$ " appear on lines of a proof, " $Q\lor S$ " can be placed on a subsequent line.

Formal notation

The constructive dilemma rule may be written in sequent notation:

$(P\to Q),(R\to S),(P\lor R)\vdash (Q\lor S)$

where $\vdash$ is a metalogical symbol meaning that $Q\lor S$ is a syntactic consequence of $P\to Q$ , $R\to S$ , and $Q\lor S$ in some logical system;

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

$(((P\to Q)\land (R\to S))\land (P\lor R))\to (Q\lor S)$

where $P$ , $Q$ , $R$ and $S$ are propositions expressed in some formal system.

Variable English

If P then Q. If R then S. P or R. Therefore, Q or S.

Natural language example

If I win a million dollars, I will donate it to an orphanage.

If my friend wins a million dollars, he will donate it to a wildlife fund.

Either I win a million dollars, or my friend wins a million dollars.

Therefore, either an orphanage will get a million dollars, or a wildlife fund will get a million dollars.

The dilemma derives its name because of the transfer of disjunctive operants.

References

↑ Hurley, Patrick. A Concise Introduction to Logic With Ilrn Printed Access Card. Wadsworth Pub Co, 2008. Page 361
↑ Moore and Parker
↑ Copi and Cohen

This article is issued from Wikipedia. The text is available under the Creative Commons Attribution/Share Alike; additional terms may apply for the media files.