The Law of Non-Contradiction. Try going against this law and you may find how figured-out, in the bag, dusted and done it is. Tremendously useful.
Unless you're dealing with Intuitionistic logic:
Semantically, intuitionistic logic is a restriction of classical logic in which the law of excluded middle and double negation elimination are not admitted as axioms.
I was just wondering. Human minds are messed up in 1001 ways, but are there a few rational principles that most people already have down? Of course, the answers to this question are probably so extremely obvious that I haven't even considered them. But I ask all the same.