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I often find that my understanding of the world is strongly informed by a few key concepts. For example, I've repeatedly found the concept of opportunity cost to be a useful frame. My previous post on privileging the question is in some sense about the opportunity cost of paying attention to certain kinds of questions (namely that you don't get to use that attention on other kinds of questions). Efficient charity can also be thought of in terms of the opportunity cost of donating inefficiently to charity. I've also found the concept of incentive structure very useful for thinking about the behavior of groups of people in aggregate (see perverse incentive).
I'd like people to use this thread to post examples of concepts they've found particularly useful for understanding the world. I'm personally more interested in concepts that don't come from the Sequences, but comments describing a concept from the Sequences and explaining why you've found it useful may help people new to the Sequences. ("Useful" should be interpreted broadly: a concept specific to a particular field might be useful more generally as a metaphor.)
Moral Foundations theory (all moral rules in all human cultures appeal to the six moral foundations: care/harm, fairness/cheating, liberty/oppression,loyalty/betrayal, authority/subversion, sanctity/degradation). This makes other people's moralities easier to understand, and is an interesting lens through which to examine your own.
The Big Five Personality Traits - though I've heard these don't seem to fit non-Westerners very well. Probably still useful when thinking about Westerners. (For example, when evaluating someone as a romantic partner or a business partner in some risky venture, I find it useful to deliberately consider their neuroticism. Or when considering suggesting someone try traveling or anything adventurous, their Openness To Experience is probably relevant.)
A teleological, non-reductionist worldview, supposedly traceable through Plato, Aristotle, Augustine, and Aquinas is wrong, but is a useful concept to be aware of because some people think it's correct. It's related to why some people, particularly some religious people, oppose homosexuality. Edit: I should add that I'm not recommending an in-depth study of this concept, just reading a few blog posts on it, and then more if it's interesting to you or if you really need to engage with believers for some reason.
Any chance you have a source for more information on that? Seems interesting.