Factor Principles/Disjunction on Left/Formulation 2

Theorem

$\vdash \paren {p \implies q} \implies \paren {\paren {r \lor p} \implies \paren {r \lor q} }$

Proof

By the tableau method of natural deduction:

$\vdash \paren {p \implies q} \implies \paren {\paren {r \lor p} \implies \paren {r \lor q} } $

Line		Pool	Formula	Rule	Depends upon	Notes
1		1	$p \implies q$	Assumption	(None)
2		1	$\paren {\paren {r \lor p} \implies \paren {r \lor q} }$	Sequent Introduction	1	Factor Principles: Disjunction on Left: Formulation 1
3		1	$\paren {p \implies q} \implies \paren {\paren {r \lor p} \implies \paren {r \lor q} }$	Rule of Implication: $\implies \II$	1 – 2	Assumption 1 has been discharged

$\blacksquare$

Also see

• This is an axiom of the following proof system:
  • Definition:Hilbert Proof System/Instance 2

Sources

• 1964: Donald Kalish and Richard Montague: Logic: Techniques of Formal Reasoning ... (previous) ... (next): $\text{II}$: 'AND', 'OR', 'IF AND ONLY IF': $\S 5$: Theorem $\text{T56}$