Laws of economics
Stein's first law of economics:
When someone tells you, this time it's different, sell. If they advance esoteric theories to back up their statement, sell at once!
Stein's second law of economics:
When a government starts talking about repaying the entire national debt in the immediate future, budget deficits are just around the corner.
Stein's law on the motivation of human behaviour:
We want more, better and free!
For more laws, check out the Murphy's Laws site.