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Constructive dilemma

A valid form of logical inference in propositional logic, which infers from two conditional and a disjunct statement a new disjunct statement.


The constructive dilemma can be seen as a combination of two Modus Ponens which are connected by a disjunct statement.

The term “dilemma” in this context should be understood as a “decision” between two conditionals.


An example for a constructive dilemma could be:

If the sun shines tomorrow, [then] we will go to the beach.
If it rains tomorrow, [then] we will go to the museum.
Tomorrow the sun will shine, or it will rain [or both].
Therefore we will tomorrow go to the museum or to the beach [or both].

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