"What are the simple concepts that have most helped you understand the world?"

That question on AskMetafilter yielded 100+ answers.

Samples:

Occam's Razor - the simplest explanation is usually correct.

We want the facts to fit the preconceptions. When they don't, it is easier to ignore the facts than to change the preconceptions.

When I realised that the vast majority of other people were too busy worrying about their own appearance/conversation topic/speaking voice to judge mine, and that random waiters, tellers or passers-by would forget me within a few minutes of seeing me, it was wonderfully liberating.

By the time you have paid enough attention to a work of art to know whether it was a waste of time to take seriously, it is already too late for the answer to be useful.

Here are mine:

Any seemingly objective statement of the facts is actually slanted by the speaker's bias.

The fact that a lot of people believe something isn't a sufficient reason for you to believe it.

The fact that you live in the country you live in, or have the parents you have, is arbitrary.

Think about what's so taboo that it isn't said even though it's true; those things are especially worth thinking of for yourself, since you probably won't hear them said out loud.

Someone with a confident demeanor is trying to persuade; their demeanor doesn't prove they're correct.

The chances are slim that a whole social movement has gotten everything right.

Money is fungible. [And so are many other things.]

If the easy solution you thought of has never been successfully tried, there's probably a good reason for that.

Ask yourself why an intelligent person would disagree with you.