Laws of classical logic

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These laws of classical logic are valid in propositional logic and any boolean algebra. Some are axioms and others derived with truth tables. The logical operators ¬ 'not', ∧ 'and', ∨ 'or', the values T 'logically true', F 'logically false', and the relation ≡ 'logically equivalent to' are applied to propositions p, q, r.

Basic Principles of Classical, Propositional and Boolean Logic

* Bivalency   ¬ T  ≡  F ¬ F  ≡  T * Involution   ¬ ¬ p  ≡  p * Idempotency   p ∧ p  ≡  p p ∨ p  ≡  p Identity   p ∧ T  ≡  p p ∨ F  ≡  p (Non-)Contradiction   p ∧ ¬ p  ≡  F ¬ ( p ∧ ¬ p )  ≡  T Excluded Middle   p ∨ ¬ p  ≡  T ¬ ( p ∨ ¬ p )  ≡  F * Contraction   p ∧ ( p ∨ q )  ≡  p p ∨ ( p ∧ q )  ≡  p Commutativity   p ∧ q  ≡  q ∧ p p ∨ q  ≡  q ∨ p * DeMorgan's   ¬ ( p ∧ q )  ≡  ¬ p ∨ ¬ q ¬ ( p ∨ q )  ≡  ¬ p ∧ ¬ q Associativity   p ∧ ( q ∧ r )  ≡  ( p ∧ q ) ∧ r p ∨ ( q ∨ r )  ≡  ( p ∨ q ) ∨ r Distributivity   p ∧ ( q ∨ r )  ≡  ( p ∧ q ) ∨ ( p ∧ r ) p ∨ ( q ∧ r )  ≡  ( p ∨ q ) ∧ ( p ∨ r )

• In these logics, the starred principles of bivalency, involution, idempotency, contraction, DeMorgan, and unlisted others like p ∨ T ≡ p and p ∧ F ≡ p are traditionally derived from the remaining six above, which are considered axioms. One could reverse these derivations and make some of the derived principles axiomatic and derive some of the former axioms (like the laws of non-contradiction and the excluded middle) from them.

[edit] See also

• Laws of thought