Introduction to Logic - Ch. 4

(Originally written October 29, 2006 in Book 9)

Introduction to Logic
Harry J. Gensler

Chapter 4: Propositional Proofs

The way to start a proof is to assume the opposite of the conclusion. Then you must attempt to prove a contradiction in the assumption. If a contradiction is found then the argument is valid. If no contradiction is found then the argument is invalid.

More S-Rules and I-Rules...

• A premise is a line consisting of a well formed formula by itself
• An assomption is a line with "asm" in front of it
• A derived step is a line with the "therefore symbol" in front of it
• A formal proof is a vertical sequence of zero or more premises followed by one or more assumptions or derived steps, where each derive step follows from previously unblocked lines by RAA or one of the inference rules, and each assumption is blocked off using RAA
• Two well formed formulas are contradictory if they are exactly alike except that one is negated
• A simple well formed formula is a letter or its negation. All others are complex well formed formulas

[About three pages of logic problems/proofs]