Metamath Proof Explorer


Theorem pm4.61

Description: Theorem *4.61 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.61 ¬ φ ψ φ ¬ ψ

Proof

Step Hyp Ref Expression
1 annim φ ¬ ψ ¬ φ ψ
2 1 bicomi ¬ φ ψ φ ¬ ψ