Rigali's Law and Lovaglia's Law
If they were dictionary words, then "Rigali Lovaglia" would be a Googlewhack. But there's no good reason for it, their laws should be far more prevalent.
These are two of the most important laws you can know.
Rigali's Law : The more important the buying decision, the higher the emotional proportion will be of that decision.
and, more generally,
Lovaglia’s Law: The more important the outcome of a decision, the more people will resist using evidence to make it.