Natural Deduction Rule (¬I):
Natural Deduction Rule (RAA):
My book [Ian Chiswell & Wilfrid Hodges, Mathematical Logic (2007)]presents these two rules and then adds:
The use of (RAA) can lead to some very unintuitive proofs. Generally, it is the rule of last resort, if you cannot find anything else that works.
The first rule adds a negation sign, the other rule removes it. For some reason removing is worse then adding. Can you explain why?