The "Two Things" List: "For every subject, there are really only two things you really need to know. Everything else is the application of those two things, or just not important." Examples:

The Two Things about Trading
1. Buy low.
2. Sell high.

The Two Things about Marketing:
1. Find out who is buying your product.
2. Find more buyers like them.

The Two Things about Artificial Intelligence:
1. "Real" AI is very, very hard.
2. As soon as a branch of AI has practical applications, it ceases to be AI.

The Two Things about Economics:
1. Incentives matter.
2. There's no such thing as a free lunch.

The Two Things about Star Trek:
1. Don't beam down in a red shirt.
2. You can always talk evil computers into destroying themselves.

The Two Things about World Conquest:
1. Divide and Conquer.
2. Never invade Russia in the winter.

(Via Tom McMahon.)