I wonder how to give a formal, Fitch-style, proof of P v P ..?
I am able to prove it by contradiction when I use that P & P is a tautological consequence of (P v P ) . But in order to prove this (one of the DeMorgan's laws I think?) formally I have to use the law of excluded middle itself...
I also know how to show these tautologies with truth tables, but an exercise explicitly ask for a formal proof of P v P .
All help is appreciated