Metamath Proof Explorer

Theorem pm2.04

Description: Swap antecedents. Theorem *2.04 of WhiteheadRussell p. 100. This was the third axiom in Frege's logic system, specifically Proposition 8 of Frege1879 p. 35. Its associated inference is com12 . (Contributed by NM, 27-Dec-1992) (Proof shortened by Wolf Lammen, 12-Sep-2012)

Ref Expression
Assertion pm2.04 φ ψ χ ψ φ χ


Step Hyp Ref Expression
1 id φ ψ χ φ ψ χ
2 1 com23 φ ψ χ ψ φ χ