Natural Deduction Rules for Sentential Logic
(Revised 5 January 2011)

Note: in the following rules, the greek letters "φ" and
"ψ" are names of SL wffs.


A (Rule of Assumption)
You can write down any SL wff, depending on itself.


&I (Conjunction Introduction)
If you have derived φ and ψ,
you can write down (φ&ψ),
depending on everything φ and ψ depend on.


&E (Conjunction Elimination)
If you have derived (φ&ψ),
you can write down φ or ψ,
depending on everything (φ&ψ) depends on.


→I (Conditional Introduction)
If you have assumed φ, and you have derived ψ,
you can write down (φ→ψ),
depending on everything ψ depends on except φ.


→E (Conditional Elimination or Modus Ponens)
If you have derived (φ→ψ) and φ,
you can write down ψ,
depending on everything (φ→ψ) and φ depend on.


~I (Negation Introduction)
If you have assumed ψ, and you have derived (φ&~φ),
then you can write down ~ψ,
depending on everything (φ&~φ) depends on except ψ.


~E (Negation Elimination)
If you have assumed ~ψ, and you have derived (φ&~φ),
then you can write down ψ,
depending on everything (φ&~φ) depends on except ~ψ.


∨I (Disjunction Introduction)
If you have derived φ,
you can write down (φ∨ψ) or (ψ∨φ),
depending on everything φ depends on.
(ψ is any SL wff.)


∨E (Disjunction Elimination or Disjunctive Syllogism)
If you have derived (φ∨ψ) and ~ψ,
you can write down φ,
depending on everything (φ∨ψ) and ~ψ depend on.

If you have derived (φ∨ψ) and ~φ,
you can write down ψ,
depending on everything (φ∨ψ) and ~φ depend on.


PC (Proof by Cases)
If you have derived (φ∨ψ) and (φ→α) and (ψ→β),
then you can write down (α∨β),
depending on everything (φ∨ψ) and
(φ→α) and (ψ→β) depend on.


↔I (Biconditional Introduction)
If you have derived ((φ→ψ)&(ψ→φ)),
you can write down (φ↔ψ),
depending on everything ((φ→ψ)&(ψ→φ)) depends on.


↔E (Biconditional Elimination)
If you have derived (φ↔ψ),
you can write down ((φ→ψ)&(ψ→φ))
depending on everything (φ↔ψ) depends on.