问题
I'm trying to construct a formal proof for ((p ⇒ q) ⇒ p) ⇒ p. in Fitch. I know this is true, but how do I prove it?
I can only use And Intro, And Elim, Or Inro, Or Elim, Neg Intro, Neg Elim, Impl Intro, Impl Elim, Biconditional Intro, and Biconditional Elim.
回答1:
The following proof uses Klement's Fitch-style proof checker. Description of the symbols and the rules are in forallx. Links to both are below.
A slightly different version is on Philosophy Stack Exchange: https://philosophy.stackexchange.com/a/55395/29944 That would be another place to try to get answers to such questions.
References
Kevin Klement's JavaScript/PHP Fitch-style natural deduction proof editor and checker http://proofs.openlogicproject.org/
P. D. Magnus, Tim Button with additions by J. Robert Loftis remixed and revised by Aaron Thomas-Bolduc, Richard Zach, forallx Calgary Remix: An Introduction to Formal Logic, Winter 2018. http://forallx.openlogicproject.org/
来源：https://stackoverflow.com/questions/42838883/formal-proof-for-p-%e2%87%92-q-%e2%87%92-p-%e2%87%92-p