Theorem pm2.52

Theorem *2.52 of [WhiteheadRussell] p. 107. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Wolf Lammen, 8-Oct-2012.)

Conclusion

\( \vdash \neg (\varphi \rightarrow \psi) \rightarrow \neg \varphi \rightarrow \neg \psi \)

Proof

conax1k