Metamath Proof Explorer


Theorem axfrege41

Description: Identical to notnot . Axiom 41 of Frege1879 p. 47. (Contributed by RP, 24-Dec-2019)

Ref Expression
Assertion axfrege41 φ ¬ ¬ φ

Proof

Step Hyp Ref Expression
1 notnot φ ¬ ¬ φ