Rule of Material Implication/Formulation 2/Forward Implication

Theorem

$\vdash \left({p \implies q}\right) \implies \left({\neg p \lor q}\right)$

Proof

By the tableau method of natural deduction:

$\left({p \implies q}\right) \implies \left({\neg p \lor q}\right)$

Line		Pool	Formula	Rule	Depends upon	Notes
1		1	$p \implies q$	Assumption	(None)
2		1	$\neg p \lor q$	Sequent Introduction	1	Rule of Material Implication: Formulation 1
3			$\left({p \implies q}\right) \implies \left({\neg p \lor q}\right)$	Rule of Implication: $\implies \II$	1 – 2	Assumption 1 has been discharged

$\blacksquare$

Law of the Excluded Middle

This proof depends on the Law of the Excluded Middle, by way of Rule of Material Implication/Formulation 1/Forward Implication.

This is one of the axioms of logic that was determined by Aristotle, and forms part of the backbone of classical (Aristotelian) logic.

However, the intuitionist school rejects the Law of the Excluded Middle as a valid logical axiom.

This in turn invalidates this proof from an intuitionistic perspective.

Sources

• 1964: Donald Kalish and Richard Montague: Logic: Techniques of Formal Reasoning ... (previous) ... (next): $\text{II}$: 'AND', 'OR', 'IF AND ONLY IF': $\S 5$