Constructive dilemma  

From The Art and Popular Culture Encyclopedia

Jump to: navigation, search

Related e

Wikipedia
Wiktionary
Shop


Featured:

Kunstformen der Natur (1904) by Ernst Haeckel
Enlarge
Kunstformen der Natur (1904) by Ernst Haeckel

Constructive dilemma is 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:

<math>\frac{(P \to Q) * (R \to S), P \or R}{\therefore Q \or S}</math>

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

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 operator.




Unless indicated otherwise, the text in this article is either based on Wikipedia article "Constructive dilemma" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

Personal tools